Christmas, Easter, Nowruz, Valentine's
Both Christmas (Yule/Saturnalia) and Easter (Eostre/Ishtar) have pagan origins combined with polytheistic festivals and holidays, which over time were in turn merged into the Christian calendar.
Valentine's is certainly a Christian invention, one of many such observances across the world, but only big for marketing reasons. Also not a public holiday in any country.
Happy Nowruz, Siamak.