what is it like to celebrate holidays like Easter?
Easter is a particularly blatant case of a religious observance heavily coopted by market interests, so by the time I came along in the 1950s, it was chocolate bunnies and egg coloring (neither of which are integral to the original celebration, but do reflect centuries of pagan fertility symbols circulating around what was officially a Christian Resurrection celebration).
That’s the price one pays for living in a society where there’s money to be made off such things. We all know how commercialized Christmas is now. But do we really want to let go of our pagan tree decorating and repeats of “traditional” movies, most of which were made only after WWII: “It’s a Wonderful Life,” “A Christmas Carol” and “White Christmas” (in VistaVision!).
And then there’s Halloween (which still does carry some religious elements in many cultures). In America it has turned into functionally an over the top cosplay party with virtually no religious connotation for most people, and the secular practice has spread to England where parents grump over having to buy bags of candy to gift the marauding disguised child extortionists as do their American counterparts.
As a secularist, any chance for a good party is not to be overlooked, presuming the original celebration wasn’t too creepy to begin with (don’t look for any resurgence of Quetzalcoatl-based parties reminding us of human heart sacrifice, for example). As for Easter, its OK to bring on the chocolate bunnies.
I can't claim to know whether every Memorial Day or Veterans Day observance excludes non-believers from the official observance when religious invocations are done, but it is fair to say we aren't taken note of generally.