I would think most people are flying the 2nd or 3rd for the holiday. It might be a challenge to fly home on Sunday, though, since everyone else will likely be doing the same. Having said that, PHX is not exactly a big July 4th destination. I recommend checking the loads with airline you want to fly a little earlier than normal to get a handle on whether it is actually open or just not horribly tight so you can make plans appropriately.