It's school holidays around then right, i would have a party ON her birthday, give people enough notice so that they can take the day off work if need be, if htey can't make it during the day tell them it's a leftovers party that night

Personally you should't have to work around everyone else.
We have a similar issue with the fact DD2 is 6th Jan, everyone is generally off on holidays before school goes back or before work starts again, this year we have a friend who's son will be having a 1st b'day and a close friend will be due to pop, i don't know what were doing with hers. yet. eek