Spring and autumn are the best times to visit Manassas. April through early June brings mild temperatures and green fields — the battlefield landscape looks its best when the grass is up and the woods are in leaf. October is similarly pleasant, with colour in the trees and cool walking weather.
Summer is hot and humid, which makes the battlefield trails uncomfortable rather than dangerous, but late afternoon thunderstorms are common. The battlefield is open year-round, and the visitor center is heated in winter — January and February visits are quiet and perfectly viable, with frost on the ground sometimes adding a melancholy quality to the landscape.
The annual Civil War re-enactments at the battlefield draw large crowds, particularly around the July anniversary of the First Battle. If you want the park to yourself, avoid those weekends. The Old Town farmers market runs on Saturdays from spring through autumn.