Spring and autumn are the best months for Gettysburg: mild temperatures, manageable crowds, and the battlefield in good walking condition. May and October are particularly fine; the surrounding Adams County orchards and forests turn brilliantly in autumn.
Summer is hot and humid and busy — July especially, when the anniversary of the battle (1–3 July) draws large crowds and major living-history events. Winter is quiet, the battlefield often dusted with snow, many smaller attractions on reduced hours. The Remembrance Day weekend in mid-November, around the anniversary of the Gettysburg Address, sees parades, wreath-layings, and reenactor encampments — a good time to visit if you don't mind cold weather.