Groundhog Day is my favorite. Nothing beats roasting a whistlepig and watching Bill Murray movies all day!

Charles Darwin's birthday.

Big party with a statue of him. Then we sing a few songs while holding hands and burning crosses. "Thank you for saving us from ignorance". Then we get drunk and try to find suitable mates for progeny.

Sounds about right.

Labor Day?

This right here.  I laugh when people try and claim others are trying to take the Christ out of Christmas, when in reality it was forced upon that day.  Same with Easter, but to a far lesser extent.  They are Atheistic holidays whether Christians want to believe it or not.  

we need a Darwin Day

I just put Groundhog Day on repeat.

That's redundant.

Atheists dont believe in holy days

if you strip away all religious holidays, state based patriotic holidays, and non-religous historical or traditional holidays the one that comes to mind as being very atheistic is pi day.  Pi (3.14159..) is celebrated in march 14 (3/14) by baking and eating a pie.   I feel it celebrates my resonance with the facts and evidence found in math and science over religions resonance with faith and belief. 

there is also mole day but there isn't really an obvious way to celebrate it.

i can haz day?