Dd was born on christmas day 2002 (Slalomsuki- same day!)
We've done things differently each year so far
At first we did birthday in the morning, then a birthday lunch, and Christmas in the afternoon with Christmas dinner in the evening.
That hasn't really worked for the last couple of years, though, because when she sees her stocking from Santa, it's v. difficult to say "no, you'll have to wait 'til afternoon to open those pressies"
We now combine it all, and even open a birthday pressie or 2 on Christmas Eve, and keep some over for boxing day.
I still think she misses out a bit, though, so we only gave her a small b'day pressie last year, then bought a proper pressie (a bike) on her 'half birthday' in june.
I have to disagree with Slalomsuki about a party, though. last year, for the first time I organised a party for dd with all her friends (hired a local community hall) and although it was on the 22nd december, everyone (bar 1) attended. Parents said it filled the gap between pre-school finishing, and Christmas Day, nicely!