August is the peak season, driven by the race meet. The town is lively and buzzing, but prices at restaurants and accommodation double or triple, and everything fills up weeks in advance. If horse racing is your reason for visiting, August is obviously the time — but book early.
The rest of summer is pleasant: the performing arts season at SPAC runs through July and August, the springs are accessible, and the Adirondacks to the north are at their best. Spring and autumn are excellent for visiting the Saratoga National Battlefield — foliage in October is exceptional. Winters are cold and snowy but the town doesn't shut down; there's skiing at nearby Gore Mountain.