When I was young and even when I was first married (60s / 70s), most stores were closed on Christmas. You might be able to find a 24 hour convenience store open. So, if you had forgotten something for Christmas dinner, depending on what it was, you might be able to pick it up last minute from a convenience store. Pumpkin pie without whipped cream just doesn't make it, you know.
As Christmas became less a religious holiday and even more commercial, grocery stores started being open limited hours for Christmas. But now it seems, grocery stores have gone back to being closed Christmas Day, with some convenience stores and pharmacies open.
In Philadelphia, Acme was open until 1 pm. Giant, ShopRite, Trader Joe's, Wegmans, and Whole Foods were closed Christmas Day.