Tartu is best in late spring and early summer (May to July), when the days are long, the riverside parks are in use, and the student population is still in residence. The Student Days festival at the end of April is the single liveliest time of year — a week of events, outdoor concerts, and general revelry.
Winter is cold and dark but the city doesn't shut down — the university runs year-round, the cafés are warm, and the Emajõgi sometimes freezes enough for walking. Avoid the August lull when many students have left and some cafés reduce their hours.