Some/most closet atheists go through the motions of celebrating Christmas and many atheists enjoy the secular part of Christmas. Other atheists refuse celebrate any holidays or choose alternative secular holidays. Many atheists celebrate Christmas because they do not want to stand out as different. ||If you are on your own what you do over Christmas should be a matter of personal choice. Many places will pay you way above the usual pay rate to work over Christmas.
Lying and telling young children that Santa Claus exists can prepare them to accept religious belief.  Atheist parents living in cultures that celebrate Christmas may choose to give their children Christmas presents, of course there is no need to pretend presents are from Santa. If children feel left out when their friends get presents this can cause resentment. You don't want children to feel bad bout being in an atheist family.