They can be fun, and you don't have to celebrate them for religious reasons or bring religion into it, because they've become cultural norms with prominent non-religious elements involved (for example, Easter is supposedly about Jesus' resurrection, but we really celebrate it with Easter bunnies, eggs and candies, or Christmas is supposed to be about Jesus' birth, but it mostly centers around Santa, gifts, Christmas lights, etc.), so you don't have to subject yourself to the ridiculousness of religion to enjoy them. On the other hand, there may be a little bit of cognitive dissonance involved for atheists insofar as the religious elements are more or less unavoidable (such as people putting up nativities and Christmas music often having lyrics about Jesus' birth) or fundamental to the meaning of the holiday.

Then I guess there are religious holidays that are only celebrated by religious people, such as Hanukkah, which I don't have much opinion about other than that the religions themselves, and by extension the holidays, are fundamentally ridiculous. But I guess those holidays can be fun or rewarding for their practitioners anyway (but maybe not as fun as Christmas or Easter). But they're a shame inasmuch as they're part of the general religious brainwashing of the children who partake in them.