I forgot my mom's birthday on Monday. Course, I was working about 10 hours that day and so I legitimately "forgot" and then I forgot the next day and the next day (I'm a teacher without a planning period so I work loooooong hours most days and come home exhausted). So anyhoot, I called Saturday to ask what my parents wanted for Christmas and THEN it dawned on me that I forgot to call! And I usually am good about sending a card too, but didn't do that either. Oh well, it happens. My mom wasn't offended at all - she totally understood.
So, it happens both ways, I would just let it go and not worry about it. It's not worth getting upset about.