The undisputed peak is January and February, when half of Buenos Aires decamps to the coast. The beaches are packed, prices are highest, and the city has an electric energy; book well ahead or expect to struggle for accommodation. December and March are shoulder months — still warm enough for the beach (20–22°C water temperature) with smaller crowds and lower prices.
Outside summer, Mar del Plata is a different city. Autumn brings crisp Atlantic air, the crowds disappear, and the city reveals its working-town character. Winter (June–August) is cold and frequently foggy, but the casino, the restaurants, and the cultural calendar keep running, and some visitors specifically prefer the off-season atmosphere. The National Sea Festival takes place in February; the International Film Festival in November.