I usually go to Gettysburg for my birthday every year (July 5th), or at least the Saturday closest to my birthday, to go out to dinner (at Frarnsworth house, and maybe do a ghost tour, and there are usually re-enactors there at least thru the weekend, but this year there were 3 completely different groups of re-enactors running around, and way to many people for me or my roommate to deal with. We thought of the potential for traffic jams and decided to stay local.
It's much less crowded during the school year, even on weekends, when most of the people who are there live within driving distance, and there are not a million tourists. There's nothing wrong with tourists, but I hate crowds.
When the weather is cool, and the days are short, and the park is all but deserted, you can stand at the high water mark and almost hear inside your mind the echo's of the battle. Or standing in the field of pickets charge some can hear in their minds the crys of the wounded.