I'd still be struggling to keep up with the days of the year, not just for the holidays, but because zombies or no, we've still got to eat, and I'd be doing whatever I could to keep up some sort of crop rotation. When whatever I thought was the fourth of July came around, me and mine (assuming I had anyone with me) would celebrate the independence granted by our higher functions rather than any founding fathers, and we lay waste for a day or so. After all, what else is there for a responsible adult to do in the middle of the summer but vacation?
In our attempts to be as useful and (formerly) American as possible, I'd throw sparklers into the hands of those at the front and rear of our caravan, load folks up with fireworks and gasoline, and we'd make a little pilgrimage to the nearest stadium where baseball games were held in better days. After our sojourn and the inevitable horde that it attracted reach the right place, we'd play rope-a-dope for a while, seeding the field with the undead and the means for their inceneration. The ensuing bar-b-que that'd take place once everyone was back up in the stands with a good view might be considered an abomination of God just to look at, but hey! It's a holiday. We're letting our hair down. And honestly, what could be offensive about setting a sizable chunk of the local horde ablaze with tasty fireworks anyway?
The far more discreet walk home would still have a livelier spring in its step, I assure you.