Washington DC is one of the few American cities where the most famous outdoor settings — the National Mall, the Lincoln Memorial Reflecting Pool, the Tidal Basin during cherry blossom season — are publicly accessible for small ceremonies but require National Park Service permits for organized events. The permit process is straightforward for small groups and well worth the effort: the visual payoff of exchanging vows in front of the Jefferson Memorial at dawn or beside the Lincoln at dusk is simply unavailable anywhere else.
For couples who want to avoid the NPS permit process, DC's roster of historic neighborhoods provides excellent alternatives: Georgetown's brick-lined streets and canal towpath, the Petworth neighborhood's craftsman-era architecture, Rock Creek Park's wooded gorge within the city. Private venues in the Shaw, Navy Yard, and Logan Circle neighborhoods cover the range from converted industrial lofts to historic townhouses.
DC's geography straddles Maryland and Virginia; some couples specifically choose to cross into Virginia or Maryland for a ceremony if logistics there are simpler. The climate is Mid-Atlantic: hot humid summers (avoid midday July–August), spectacular cherry blossom peak in late March–April, and warm falls through October.