If you're running on the theory that we shouldn't celebrate any man-made Holidays, then we're pretty much never gonna celebrate. I suppose you could adopt Jewish traditions and celebrate Passover, et al, but good luck convincing the rest of us.
I mean, look at major US Holidays:
New Years? Completely different day than the Jewish new year.
Valentine's Day? Name taken from a man, a Catholic Saint, with pagan iconography in the form of Cupid
thrown in for good measure.
Easter? Pagan origins of the name already covered. Christians have always celebrated Christ's resurrection, but the early church did it every Sunday, not once a year.
Independence Day? Instituted to celebrate the emergence of a nation that doesn't force it's citizens to follow a state church! How horrid!
Halloween? Instituted to replace pagan harvest festivals with celebration of the Saints.
Thanksgiving? Sure it's made to give thanks to God, but it was man-made, signed into law by old Honest Abe.
Christmas? No indication that the early church celebrated Christ's birth. Modern Christmases tend to be more about crass commercialism than about Christ's birth anyway.
So you can either choose to live in paranoia about the holidays of dead religions, or you can join the 21st century and learn to have a little fun.