All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is impermissible to congratulate the non-Muslims on the occasion of their festivals and their religious rituals. Ibn al-Qayyim, said in his book Ahkaam Ahlu Ath-Timmah: “With regard to congratulating (non-Muslims) for the rituals of disbelief that is specific to disbelief, then this is prohibited according to the agreement of all the scholars, such as congratulating them on their festivals and their fast by expressing good wishes to them saying: “Happy feast or enjoy your feast and the like.” [End of quote]
However, merely congratulating them on their festival, even if it is forbidden, it is not disbelief or apostasy.
Allah Knows best.