October is the season and it is not close. Autumn weekends here stack three things on top of each other — leaf colour on the ridges, the anniversary of the raid on the 16th to the 18th, and Halloween — and the town's parking, restaurants and tour capacity all reach their limit at the same time. If you want an October Saturday, book it weeks out and come down in the afternoon.
Spring and summer run on longer daylight, which pushes start times later. A June tour cannot begin at dusk without starting after half past eight, so summer walks tend to begin while it is still light and finish in the dark. That is a different and slightly less atmospheric experience, and it is considerably easier with children.
Winter thins out sharply. Some operators pause entirely, some run weekends only, and the town around them contracts at the same time — kitchens on High Street and up in Bolivar cut back to weekend trading between January and March. A January evening here is cold, dark by five, and genuinely quiet. Some people find that better. Confirm the tour is actually running before you drive out.
Anniversary weekends are worth knowing about in both directions. Mid-October brings the raid, mid-September brings the 1862 siege and Antietam twenty minutes north, and both fill the town with people who came for the history. Ghost tours sell out on those weekends too.
Two nights ahead is enough booking for most of the year. Two weeks is not too much for an October Saturday.