Summer (June–August) is the peak season and with reason. Days are very long — Riga is far enough north that it barely gets dark in June — the outdoor café culture is in full swing, and the city is at its most lively. The Old Town gets crowded; book accommodation well ahead.
Midsummer (Jāņi, 23–24 June) is the most important holiday in the Latvian calendar: a pagan festival of bonfires, singing, flower garlands, and enormous quantities of cheese and beer. Many Latvians leave the city for the countryside; the ones who stay throw parties. The Ethnographic Museum hosts a major midsummer event.
Spring and autumn offer the same sights with far fewer tourists and lower prices. September in particular is pleasant — warm enough, not crowded, with the autumn colours beginning in the parks.
Winter (December–February) is cold and dark, but Riga does Christmas well. The Christmas market on Doma laukums is one of the better ones in the region, and the city claims (with some justification) to have invented the decorated Christmas tree. Temperatures regularly drop below zero; the river sometimes freezes.
The Riga City Festival in August fills the streets with concerts, markets, and events, and is worth timing a trip around.