"2 months ago". There are people that have total recall, but that is very rare. What you refer to is a "passing thought" which is an idea that comes to mind in a circumstance or situation that is then forgotten later. It is why we put things on calendars and schedules so we remember.
The new thought from your boss is that it was your birthday and you should have the day without work, and it is even better if he gives you pay (salary) for the day and makes it a paid holiday this year for you.
Nothing is wrong. You're good. It would be a bit rude but within acceptable to immediately have said something courteous, but forgiving like-
"Oh. Thanks for the day off. I seem to remember about an employee pizza party plan to come in for, that you mentioned a couple of months ago, but I could use a day to go shopping and relax at the pool. See you tomorrow." That would have given him the chance to remember- a gentle tap reminder.
It is important to stay on good terms and relationship with your boss/manager, but you are allowed a little leeway done carefully. It was his mistake, and you took the statement a couple of months ago as more important of course.
Another better option was a working day before your birthday to nicely remind him of what he said.
"It's my birthday tomorrow. You mentioned a couple of months ago about a pizza lunch to come in for. Is it still on, or should I take the day off, or regular work day?"
In any case, you have nothing to be weird about a misunderstanding. They happen.