Happy Vesak (can or cannot?) Day

Since it has been a practice, I would like to wish my Buddhist friends 'Happy Vesak Day' as to symbolise my respect to them and their religion, without my faith as a Muslim ripped by it.

It is our Malaysian culture, and I grew up with it. Is it a sin to utter 'Merry X'mas' 'Happy New Year', 'Gong Xi Fa Cai' or 'Happy Deepavali' to the non-Muslims?

I don't care!

The picture about shows some Muslim women in Indonesia joining their Buddhist followers releasing a lantern into the air at Borobudur on Vesak Day.