Christmas was NOT when Jesus was born. Jesus was born in the summer. Do you know of sheepherders that herd their sheep in the winter? Neither do I. Hence Jesus was born in the summer.

Christmas was a Pagan holiday celebrated by the Romans for the Pagan god Mithras. December 25th is HIS birthday and Christmas began to be celebrated at Constantines order when Constantine converted to Christianity and ordered Christianity to become the official religion of the country.

As for why those other holidays are celebrated....

Valentines day, do you remember cupid? Cupid is this cute little Pagan god with a bow and arrow that went around shooting people with it to fall in love. Do you, or do you not, see his image EVERYWHERE on Valentines day?

Valentines Day is Pagan too. It was just twisted by the Christian religion like all the other holidays were.

Oh but then... what about Easter? Well, lets look at this shall we? Thats the Pagan holiday of Oester celebrated for the turning of the seasons. Again, perverted and twisted by Christianity.

Every single one of those holidays are Pagan in origin and have existed long long before Christianity twisted them to suit its own purposes.

Now that we've established that there is a reason for non-Christians to celebrate those dates....

Atheists celebrate them for fun. To an Atheist the holidays are about family and loved ones. Its the one time of year that everyone has off, everyone can be together, and everyone can enjoy.

You really need to do proper research instead of making false claims. The only person you have to blame for your ignorance is yourself and you should know better.

YOU are just in denial because you don't like the answers you're getting.

Edit: The chocolate egg is secular, the Christmas tree is actually the Yule tree and is pagan, fireworks are Chinese. So what? They're all Pagan or secular.