As the answers in How can I create original holidays for my world? discuss, humans in the "early days" of humankind had no science, so for questions like "why is winter coming?", religious beliefs were the only things they had. Those questions were quite important to a mostly agricultural society, so they became central to their religions and important dates. Religious holidays still reflect this: Christmas is in the middle of winter (originally the longest night in winter), Easter aligns with celebrations of spring. Thanks giving is a harvest festival.
Those agricultural dates are unimportant in a society where I can buy strawberries in December (shipped in from Australia or New Zealand). We still celebrate Christmas (or Easter, for that matter), but I think that for many people this is not for religious reasons, but just because they have always known it like this and everyone else does.
Now fast forward to a near-future setting, say, a hundred years from now. A third world war, accompanied with mass-immigration, mixed humankind pretty nicely. With people from different cultural backgrounds living in the same city, the "every one else does it" part of the story is not true any more. Also most people are atheist, so the religious meaning will be irrelevant for most people. Globalisation works as it does for us, so seasons have almost no effect on the availability of food.
What holidays/important dates does this society have? Would there still be celebrations for Christmas?