Washington DC in May: 25°C / 77°F afternoons, 15°C / 59°F mornings, 11 rain days — late spring, Smithsonian Folklife Festival end of month, Memorial Day.
May in Washington DC is late spring transitioning into summer. NOAA NCEI data for Reagan National (DCA) put afternoons at 25°C / 77°F, mornings at 15°C / 59°F, 11 rain days totalling ~95mm, and 14h 25m of daylight. The wardrobe register is preppy-political — DC is among American cities that still rewards a blazer at lunch and a button-down at dinner, particularly in Georgetown, Capitol Hill, and Dupont Circle. Local design vocabulary: Brooks Brothers (founded 1818 NYC, the Georgetown flagship has dressed five US presidents — Lincoln, Roosevelt, Kennedy, Reagan, Obama), Tory Burch (Pennsylvania Avenue store), Ann Taylor (multiple DC locations, the political-staffer mainstay), J.Crew (Wisconsin Avenue Georgetown), Vineyard Vines (Pennsylvania Avenue, the DC summer-prep register). The Smithsonian Folklife Festival opens late May on the National Mall (typically the last weekend of May through July 4). Memorial Day weekend (last Monday of May) closes the month with the Rolling Thunder veterans' motorcycle ride through DC, the National Memorial Day Concert on the West Lawn of the Capitol, and reduced museum hours. Georgetown cobbles (M Street and Wisconsin Avenue's brick pavement) reward leather loafers; stilettos struggle.
Washington DC's preppy-political register draws from Brooks Brothers (founded 1818, NYC original but Georgetown flagship dressed five US presidents), Tory Burch (Pennsylvania Avenue store), and L.L. Bean — the East Coast establishment uniform with a DC sharper edge.

DC May base. Brooks Brothers, Vineyard Vines, J.Crew, or international Sunspel and COS. Stock 3+ for café-and-Smithsonian rotation.

The DC layer. Brooks Brothers cashmere cardigan, Theory blazer, or Vineyard Vines linen blazer. Layers for 15°C / 59°F mornings and AC-aggressive Smithsonian galleries.

Two bottoms. Jeans for Georgetown café days; chino trouser for Capitol Hill and recognized restaurants. Brooks Brothers Madison-fit chino, J.Crew Wallace, or Theory polished slim.

For dinner reservations and Smithsonian openings. Tory Burch (Pennsylvania Avenue), Ann Taylor, or J.Crew Cécile collection.

The DC shoe. G.H. Bass Weejuns penny loafers (heritage East Coast), J.M. Weston, or Sam Edelman pumps. Skip stilettos on Georgetown brick (M Street) and the Smithsonian's marble plazas.

11 rain days. Burberry trench, J.Crew packable rain shell, or L.L. Bean Stowaway rain jacket. The DC trench register is real — political-staff-shorthand for considered.

Tory Burch, Coach heritage, or Mansur Gavriel. The DC bag reads structured-leather over slouchy.

May UV in DC reads 7-8 (high) on clear afternoons. Persol or Saint Laurent sunglasses; a light cotton or modal scarf for AC museum interiors.
Cotton tee · chino trousers · loafers · cardigan · trench · structured bag. Coffee at Compass Coffee 8am, Smithsonian Air & Space or American History 10am-1pm, lunch at Old Ebbitt Grill.
Cotton blouse · chino trousers or knee-length dress · low-heel pumps · blazer · structured bag. Dinner at Le Diplomate (14th Street), Rasika, or Rose's Luxury (Capitol Hill) 8pm; drinks at Off the Record (Hay-Adams) or Roofers Union after.
A suggested look — Washington DC May feminine walking look: white tee, cream cardigan, khaki chinos, navy trench, black loafers.
Per NOAA NCEI data for Reagan National (DCA): average daily high is 25°C / 77°F, low is 15°C / 59°F, 11 rain days totalling ~95mm. Daylight is 14h 25m. May is DC's late-spring shoulder month — afternoons climb toward summer humidity (50-60%) by late month; mornings still drop to 13-15°C / 55-59°F. Cherry blossom season (typically late March to early April) is over; the National Mall's tourism crowd shifts toward Smithsonian-and-monument-focused.
Smart-casual to smart, depending on context. Capitol Hill staffers and lobbyists wear suits Monday-Thursday (lighter weights in May); Friday casual register allows chinos + button-down + loafers. Smithsonian and museum visitors run smart-casual: chinos or jeans + button-down or fine-knit + loafers or low-heel pumps. Recognized restaurants (Le Diplomate, Rasika, Rose's Luxury, Minibar, José Andrés' Zaytinya and Jaleo) enforce smart-casual at minimum. Skip athletic shorts, flip-flops, ripped denim, oversized graphic tees outside Adams Morgan or Shaw nightlife.
The Smithsonian Folklife Festival is the National Mall's annual outdoor cultural showcase, running 10-12 days from late May through July 4 (varies year to year). Each year features 2-3 country or theme programs (recent years have featured The Ozarks, Northeast Asia, Catalonia, Peru, Armenia). The festival draws 1 million+ visitors over its run. Pack: smart-casual layered (cotton tee + fine-knit cardigan + chino trousers + leather loafers), packable rain shell, water bottle, SPF 30 (the Mall has limited shade), comfortable walking shoes for the multi-acre festival footprint.
Brooks Brothers (Georgetown flagship, has dressed five US presidents — Lincoln, Roosevelt, Kennedy, Reagan, Obama), Tory Burch (Pennsylvania Avenue), Ann Taylor (multiple), J.Crew (Wisconsin Avenue Georgetown), Vineyard Vines (Pennsylvania Avenue), Loro Piana (Pennsylvania Avenue), Hermès (DC Galleries at Tysons or 14th Street). City Center DC (Pennsylvania Avenue) holds the international luxury cluster (Hermès, Louis Vuitton, Loro Piana, Dior, Tom Ford). M Street Georgetown holds J.Crew, Theory, Reformation, Aritzia, Madewell. Mass Avenue Heights holds Sherman Pickey (Georgetown's heritage tailor since 1971).
Smart-casual respectful. The National Memorial Day Concert on the West Lawn of the Capitol (Sunday before Memorial Day) draws 25,000+ in folding-chair-and-blanket setup; pack cotton tee + chino trousers or knee-length dress + cardigan + leather loafers + structured leather bag. The Memorial Day Parade on Constitution Avenue runs Monday morning; standing-and-walking smart-casual works. Arlington National Cemetery wreath-laying ceremony at the Tomb of the Unknowns: covered shoulders + knees, dark colors (navy, charcoal, black), polished closed-toe shoes. Skip athletic wear, flip-flops, baseball caps inside Arlington's tomb area.