I hardly ever make reservations in the NE (PA/MD/NJ/NY/DE) as we usually "wander" with no specific destination.
We have never had a problem getting a site from Mon->Thu but I have found that the week-ends are getting filled more than in the past. Now however, I will book 24 hours ahead for a State Park in the general area we think we will end up in.
What I have found odd, is even though ReserveAmerica shows no availability, less than half the sites are occupied. This has happened several times last year.
We usually stay home during the holiday week-ends.