How is it celebrated?
My neighbors and I celebrated yesterday by going to Dairy Queen and getting Blizzards. (Mine was pumpkin pie -- a fall special and highly recommended if you live near a DQ; it has pieces of actual crust in it, though I keep forgetting to ask them to hold the whipped cream.) But we do that with every holiday.

Otherwise, I suppose it's celebrated about the same way Martin Luther King Day and Juneteenth are celebrated -- people briefly discuss the origin of the name, in some cases probably argue over whether it should be a national holiday, and otherwise go about their ordinary lives.