it is pretty much a toss up. I find that on most holiday weekends it is actually easier to get the clients out of the house, since on one of those days they themselves probably have plans, this holds true for tenants also.
As far as July the 4th vs. Labor Day. In our area the influx of people is usually those who are visiting relatives and buyers who are still looking and if they have kids trying to get settled prior to the school year commencing.
In our area (Southwest Riverside, California) a lot of people head up to the River and/or Vegas. However, this is offset by those who do not want to be caught up in the last summer holiday weekend and they stay closer to home or visit wity relatives. In addition, many buyers, may have already made a decision during the earlier summer months.
As for me, will I be holding an Open House this weekend ? Absolutely, I choose to be optimistic, I know the odds are minute of someone actually putting an offer on the house I am show casing, but, I only need one. Since I don't know where that "one" is, I am willing to be out there just in case.
If Realtors wanted a sure bet, then we wouldn't be in Real Esate:)
Have a Good Holiday Weekend