Summer (June–August) is peak season and the most rewarding time to visit. Temperatures reach 20–25°C, days are long (nearly 18 hours of daylight around the solstice), and Riga's outdoor cafes and bar terraces come alive. The single best reason to time a visit around late June is Jāņi — the Midsummer festival on the night of 23–24 June. Latvians escape to the countryside, light bonfires, drink beer, eat caraway cheese, sing folk songs, and attempt to stay awake until sunrise. It's one of the most genuine traditional festivals in northern Europe. Book accommodation well ahead if you're coming for Jāņi.
Spring (April–May) and early autumn (September) are quieter, cheaper, and still pleasant. Riga is not overrun, prices are lower, and the Gauja forests look their best in spring and autumn colour.
Winter (December–February) is cold — often below freezing — and dark, with only 7–8 hours of daylight. But Riga's Christmas market is one of the better ones in the Baltics, and the city has a certain atmospheric quality under snow. The bobsled track in Sigulda operates in winter for real ice runs.
Avoid late July and August in Jurmala unless you enjoy crowds — it gets packed with Latvian and Russian holidaymakers.