Spring and fall are ideal. October brings fall foliage to the hills surrounding the river confluence, and the light on the rivers in the early morning is extraordinary. May is pleasant for the Appalachian Trail hiking season. Summer weekends are crowded and hot; the town's narrow streets fill with day-trippers and the parking situation becomes difficult. Weekdays in any season are far more enjoyable.
The town can be done as a day trip from Washington or an overnight stop. The surrounding area — Shenandoah National Park is about 60 kilometers south, Antietam Battlefield is 20 kilometers north in Maryland — makes Harpers Ferry a logical center for several days of Civil War and landscape exploration.