The problem is that July 4th is one day. Not a month. About all July 4th pictures can be posted within 2-3 days, giving time for post processing (except for those on a vacation for a week, and out of town or country).
Even on broader subjects, a month may be too long for most people? I don't know. People tend to get bored fairly easy, when something goes on for a long time. I've seen forums that had POTM (Photo Of The Month) contests, that had pretty decent turnouts. But this isn't a contest. Also, the POTM contests usually allow just one entry per person, so you're not bombarded with a lot of photos from each person.
Most contests that the only thing you win is bragging rights, are Weekly Challenges. At least the ones I've seen were. I remember awhile back, someone tried to start one of those here, and it didn't go over so well. I'm a member of another forum that does a Weekly Challenge. During the week, you can post more than one picture that fits the subject, but before voting time, you must tell which picture is your entry. At the end of the week, a poll is set up with the pictures in it to vote on them. You have 24 hrs. to vote, and the poll closes. The one with the most votes chooses the next weeks subject, and sets up the poll for that weeks voting. It averages 5-6 entries each week. We have had as many as 10 before. The Challenge week runs from Thursday to Wednesday evening, which the poll is set up then. That way, voting is during the week, while everyone is still around. Some are gone on weekends.
I don't know what the answer is. You know it's hard to please everyone. Someone will always dislike the way it's being done. I think the challenge works pretty well at the other forum, because it's a lot smaller forum, so there's no so many temperaments to deal with. Good luck figuring it out................
EDIT:
Also, in the challenge, the photos must be taken during the week of the challenge.