A. Keep celebrating it on the day you're used to.
B. Switch to the new date. Everyone else is doing it.
C. Celebrate both, but the old one more.
D. Celebrate both, but the new one more.
E. Other in comments.
As an example, we'll use Christmas. In the US it's December 25th while in Russia it's January 7th. If you move to Russia, would you switch the date or keep the US one?