"Christmas or Christmas Day (Old English: Crīstesmæsse, literally "Christ's mass") is an annual commemoration of the birth ofJesusChrist,[5][6] celebrated generally on December 25[2][3][4] as a religious and culturalholiday by billions of people around the world. A feast central to the Christianliturgical year, it closes the Advent season and initiates the twelve days ofChristmastide.[7] Christmas is a civil holiday in many of the world's nations,[8][9][10] is celebrated by an increasing number of non-Christians,[1][11][12] " - Wikipedia
It is at its core a religious holiday. You obviously don't have to be Christian to celebrate it. Whether you're an atheist or not isn't really a big deal. It's pretty much fact that Jesus Christ existed and I think everyone is free to celebrate his life/birth regardless of whether you believe he was God the Son incarnated or that he came back from the dead. Or, you can just look at it as a time to celebrate being with loved ones and exchanging gifts.
Someone telling you what you can and cannot celebrate is probably just so full of their own bullshit opinion that their judgement is clouded.