What makes a birthday?

The monkey had a friend party on Saturday, November 20th. He counted down all week, and decided it was his birthday because of the party. He even sang Happy Birthday to himself. When I pointed out that he was born on the 24th, he decided that was his "real" birthday. We went to see Tangled on his birthday, and I gave him the first of his gifts. By bedtime, he decided that he didn't turn six until he had his family party, which isn't until this Sunday. I think it is very confusing to the young as to when birthday's actually happen since you can't celebrate with a party on a Tuesday night during school time. A bit easier for summer birthdays in that respect, but still difficult.

The day you are born?

Or the day that you celebrate?

What do you think makes the memory? Is it a fixed date or simply the celebration of any important event?