When is atheist day?

there is no atheist day.

atheists usually celebrate the local traditional holidays in a secular way

for example my family celebrates Christmas with a xmas tree, gift giving and a big family lunch - this celebration has no religious significance or elements it is simply a time to get together with family and enjoy each others company

I think other atheists in other traditions do the same thing

by the way Atheism is characterized by a lack of belief in a god or gods so atheists don't worship anything