“No memory is ever alone; it’s at the end of a trail of memories, a dozen trails that each have their own associations.” ~ Louis L’Amour (1908-1988)
Have you ever pondered the idea that the success of your man’s remembering important details has connection with how he meaningfully relates those details to other things and how his brain reconstructs the past?
I am so far from being an expert on romantic relationships, but I like to take a stab at this topic, periodically.
I know this must make you feel doleful and ill-tempered. “All week, I told him to not forget about my mother’s birthday party and how important it is to her that we be there. He still forgot about it and scheduled a meet-up that night with his friends! Does he not care about my parents? Does he not care about anything I say?” This is just one example of the incalculable things a man could forget. Right?
As a single man who has disappointed a lot of female friends and past girlfriends, I can certainly attest to this. I have often felt deep remorse over my forgetfulness. It is not caused by intentional disregard…at least, I hope. Men can be so forgetful that one should wonder how any of us could remember our heads if they were not attached to our bodies. My mother used to always express this witticism to me when she felt aggravated.
I don’t want to make dumb excuses for carelessness, but I do want to seek a factual explanation as to why my side of the human species is so prone to forgetting. And I hope that more women out there can see us men as redeemable.
First, let’s begin with defining memory:
Memory is the rebuilding of past experiences via the synchronous discharges of many nerve cells corresponding with whatever original experience(s) you are thinking of. This involves encoding, storing, and subsequent retrieval of information that is gathered from things we have seen, heard, smelled, touched, and tasted. However, memory is like a collage or jigsaw puzzle that you must piece together. It is not like books discretely shelved in a library for facile access. Inside your brain are reconstructions using various elements scattered throughout it. 
In memory studies, we have come a long way since the days of when behaviorists believed that memory was a “single, simple system.”  Though the computer analogy may lack some explanatory power pertaining to human memory, we still have attained great discoveries and new levels of understanding from analogizing that memory is a kind of information-processing system.
Okay. Moving on…
We have short-term and long-term memory. The statement that you and your husband are expected to show up to your mother’s birthday party is stored in the sensory memory before hopefully moving to the short-term memory. Whether it gets transferred to long-term memory depends on several factors. Nevertheless, remembering this will never be akin to the deep-seated memorability of how to ride a bike, or how to use the English language, or stating the name of the president of the United States.
When discussing short-term and long-term memory, Hermann Ebbinghaus is the first to come to mind, because he was a pioneer in memory research. He discovered the forgetting curve (memory is vitiated over time as attempts are not made to strengthen it) and the spacing effect (learning is greatened when you study throughout a span of time instead of cramming it all in at once). 
Ebbinghaus experimented with himself, using 2,300 boring, meaningless consonant-vowel-consonant segments such as “WID”, “ZOF” and “KAF.” He tested himself to see what he could remember, looking at each syllable for half a second and pausing for 15 seconds before looking at the list again. He tested his speed of learning and forgetting based on different time intervals and lengths. He delineated his results on a graph. Hence, the forgetting curve. And he discovered that he was more apt to remember meaningful content more than meaningless content, but memorization could increase with repetition of the meaningless content. 
Now, Ebbinghaus’ experimental consonant-vowel-consonant content may have been meaningless and that certainly cannot compare to the important things you want your boyfriend/husband to remember. I surely do not want to downplay the significance of what you want to tell your man. But the unfortunate part of mental reality is that things are susceptible to blurring and running together. And the forgetting curve is exponential in nature.
Memory operates optimally at the time of learning new information. And no matter what the content is and its level of importance, the retention decreases rapidly within days. Memory also works best without overexerted mental efforts (overlearning) to remember something. There is such a thing as storing something too strongly “and thus the effects of forgetting curve for overlearned information is shallower.”  This could be why the love of your life is extra prone to forgetting if he feels you are nagging him.
Not all forgotten pieces of information follow the forgetting curve due to other factors likely at play. Maybe your beau was having a stressful work week when you discussed with him the upcoming party. Maybe he was sleep-deprived. Maybe a lot of cacophonies or racket (e.g. children, pets, neighbors fighting, working blender, and other distractions) were occurring at the time of your talking. Environment is always a key factor to consider.
Always remember to never project your presumably unimpaired capabilities onto your man. Never assume that his memory strength is parallel to yours. Avoiding this assumption can perhaps enable you to have more empathy and mercy for him.
The U.S. National Library of Medicine published a 2011 report titled “Sex differences in how stress affects brain activity during face viewing.” Here is what I find to be salient from the report: 1) Stress has an important bearing on emotional perception. 2) Stress will cause your man to make an exodus from a social situation whereas the stress will induce you to see social support. 3) fMRI studies revealed that a man’s brain regions (the insula, temporal pole and inferior frontal gyrus) that are responsible for construing others’ emotions had a decreased functional connectivity with the amygdala, whereas that same functional connectivity with the amygdala increased in women. 4) Therefore, a man’s emotional perception is affected by stress differently from a woman’s. Be prepared for his thinking to be off-kilter if he is under stress. 
That 2011 report predicated itself on prior findings that a male’s affiliative behavior (the intent of supporting or improving one’s individual relationships with others) decreases in response to stress whereas a female’s will increase. The report’s conclusion was the following:
“This study indicates that experiencing an acute stressor affects subsequent activity and interactions in brain regions involved in decoding and interpreting others’ facial expressions in opposite ways for males and females. These findings contribute to a growing literature showing that stress affects males and females differently.”
Now, you may grouse about some of my source material, but I feel I need to include a source that is more up-to-date than a 2011 report. And I am hard-pressed to find one as prestigious as The U.S. National Library of Medicine.
MedicalNewsToday issued a 2016 report about Boston researchers investigating how menopause and levels of estradiol sex steroids affect memory. Women, aged 45-55, were said to perform better on memory tests despite their hormonal impediments. Premenopausal women performed better than postmenopausal women, though. Women in their early post-puberty stages are said to be able to outperform men in memory tasks, still. 
Perhaps we can compare your inamorato’s memory strength to the strength of flashbulb memories, which are vivid and stick out in our minds because of consternating and personally important events.  Of course, you want your mother’s birthday party to be one of his top priorities, and thereby, very memorable. But that memorability can’t be as strong as the memorability of things like the September 11th World Trade Center terrorist attacks or Boston bombings, obviously. Those are called flashbulb memories. So, maybe, you want to make a considerable impact on your man’s memory, a bit like how the impact of flashbulb memories make a person suddenly aware, but undoubtedly not as immense and not as traumatic. Flashbulb memories are not exact representations of the past either. So, keep that in mind.
Among all the various things your boyfriend/husband is expected to remember each day, there are many distractions. Since the 1950s, psychologists have known that people forget things within seconds if they are distracted from repeating a small amount of information given to them. 
This was demonstrated in Peterson and Peterson’s study (1959) on people who were asked to perform a distracting task (counting backward by threes) after trying to remember a list of three-letter segments. Short-term memory decay was demonstrated here because the information speedily faded within 18 seconds.  They say that maintenance rehearsal (mental or verbalized repetition of information) alerts your brain to make it a goal that you remember what has been said, heard, or done. You then might ask, “Why can’t my boyfriend/husband exercise that memory-making trick out of respect for me?” He could, and, again, not to make excuses for negligence, but the brain is more complicated than we want to assume. It is messy and sloppy.
Ebbinghaus’ list of nonsensical syllables, and later measuring how he recalled them, generated the revelation that things at the beginning (primacy effect) and things at the end (recency effect) of a list are more memorable than those in the middle.  Did you briefly discuss the obligatory birthday party attendance in the middle of a string of other things you discussed with your beloved man? Perhaps you should discuss the matter with him for a long duration instead of cursorily mentioning it in passing. This way he is alerted to the level of importance of what you want him to remember.
The serial-position effect that came about represented two distinct memory systems at work. As someone reads a list, the first items provoke more thinking than the items later in the list. Hence, they are stored in a repository, meant for Brobdingnagian amounts of information, called long-term memory. And the last items are stored in a repository called short-term memory, with 18 seconds as the maximum time for holding information, as you actively work with the information set before you. A growing list of items and items presented quickly will diminish the primacy effect. Slowly presented items are a booster for the primacy effect. What you are currently conscious of is contained in the short-term memory and will be transitioned to long-term memory depending on how your brain judges its need for being used in the future. Your brain remembers your coworkers’ name maybe because of the intrinsic need to avoid the embarrassment of continually forgetting his/her name. 
Sounds (acoustic encoding) and images (visual encoding) are less memorable than words programmed based on their meaning (semantic encoding).  This might explain why important things that are expressed with caterwauling and violent gesticulations, and/or any other forms of aggression are often forgotten. Your sweetheart is trying to distance himself from the cringeworthy feeling that is evoked with remembering what you said and how you acted. A gentle, affable tone would do better to produce the strong memory you wish to implant in him.
And if you would let me digress a bit, perhaps the Bible gave us some early insights on the matter:
“A soft answer turns away wrath, but a harsh word stirs up anger.” ~ Proverbs 15:1
“There is one whose rash words are like sword thrusts, but the tongue of the wise brings healing.” Proverbs 12:18
“A hot-tempered man stirs up strife, but he who is slow to anger quiets contention.” ~ Proverbs 15:18
“A fool gives full vent to his spirit, but a wise man quietly holds it back.” ~ Proverbs 29:11
Perhaps we can say that what you remember is based on how you remember it and how it made you feel. Your fives senses act as the conduit through which your sensory memory fleetingly keeps enormous amounts of data. To repeat what you already know, memory works in three stages and sensory memory is stage #1. Passing from the sensory stage (stage 1) to the next processing level is determined by how you tend to that information. Incoming sensations need some time for being distributed throughout yourself, so you can view the external world as a seamless stream instead of fragmented pieces. 
And your relationship can feel like a seamless stream as well when mercy and empathetic understanding are employed amid the struggles of life.
 Stephen L. Franzoi, Psychology: A Discovery Experience, pg. 376, Chp. 13.1, The Nature of Memory