Lithuania has a continental climate — cold winters, warm summers, and changeable weather year-round. Rain is possible at any time; bring a waterproof layer whatever the season.
Summer (June–August) is the most popular time to visit. Days are long (it stays light until after 10pm in June), temperatures reach the low-to-mid 20s°C, and the country is at its most festive. The Curonian Spit and Baltic coast beaches are in full swing in July and August. Vilnius and Kaunas fill up with tourists; book accommodation ahead.
Spring (April–May) and early autumn (September–October) are excellent times if you want pleasant weather without the crowds. City sightseeing is comfortable, prices are lower, and in September and October the forests turn gold — Lithuania is unexpectedly beautiful in autumn.
Winter (December–February) is cold and dark, with temperatures regularly below freezing and frequent snow from December through February. But Vilnius has a particularly good Christmas market, and the snowlit old town has a charm of its own. Druskininkai's Snow Arena is open for skiing in winter. January and February are the quietest (and cheapest) months.
The Kaziukas Fair in early March and Užgavėnės carnival in late February or March are worth timing a visit around. The Dainų Šventė (Song and Dance Celebration), held every four years in Vilnius, is one of the most spectacular cultural events in the Baltics — worth planning ahead for.