When to go
April, May, June, and late September through October are ideal — 70–85°F, dry, and the sky is the Mediterranean’s most reliable blue. July and August are punishing (often 95–105°F), and locals leave for the coast. November to March is cool (40s–50s) but sunny; lower hotel prices and the city is alive with locals.
Getting there
Iberia, American, Air Europa, and United run daily JFK nonstops to Madrid Barajas (MAD). About 8 hours eastbound, 9 westbound. Metro Line 8 from the airport to Nuevos Ministerios is €5 and 15 minutes. The Cercanías commuter rail from T4 to Atocha is €2.60. Taxis are flat-rate €33 from airport to city center.
Visa & entry
Schengen — 90 days visa-free for US/UK/Australian/Canadian passports.
Money
Euro, ~€0.92 per USD. Cards work most places; cash useful for tipping (round up; 5% is plenty), small tapas bars, and traditional markets. The Madrid Metro is the cheapest in Western Europe at €1.50–€2 a ride; a 10-trip card is €12.20.
What to see
Four days: Prado (a half-day; Velázquez and Goya are the must-sees), Reina Sofía (Picasso’s Guernica), Thyssen-Bornemisza (the third of the "Golden Triangle"), Retiro Park, a Sunday walk through El Rastro flea market, and tapas hopping in La Latina or Lavapiés. Day-trip to Toledo (30 min on the AVE train) for medieval architecture, or Segovia for the aqueduct and a famous suckling-pig lunch. Eat dinner at 10pm like everyone else.