Why are festivals important to many religious communities?

Religions are important to people of that religion. Christmas and Easter are important to Christianity but no other religion. Ramadan is important to Islam and no other religion. And Passover is important to Judaism but no other religion.

Why would an atheist celebrate Hanukkah or Beltain? The religious festivals are unimportant to people who are not of that religion (unless the festival has been commercialized and not celebrating it would not be socially acceptable).