Shaikh Ibn ul-Uthaymeen (may Allah have mercy upon him) said:

وإذا هنؤونا بأعيادهم فإننا لا نجيبهم على ذلك، لأنها ليست بأعياد لنا، ولأنها أعياد لا يرضاها الله تعالى، لأنها إما مبتدعة في دينه

“If they congratulate us on their festivals, then verily we do not respond to them upon that. They are not festivals for us. They are verily festivals Allah is not pleased with because they innovated in their religion” [Majmoo’ Fatawa wa Rasaail question. 404 (3/29)]

So its best not to reply. And if your able to explain the issue, how we don’t celebrate such festivals as Muslims, then this is better and a way of giving Dawah.

And Allah knows best

Faisal bin Abdul Qaadir bin Hassan
Abu Sulaymaan