Summer (June–August) is when Tønsberg is at its most alive. Brygga fills with boats and people, the outdoor restaurants and bars are busy into the evening, and the surrounding coast — the islands, fjord beaches, and waterways — is the main draw for Norwegians from Oslo and beyond. Accommodation fills up quickly in July, so book ahead. Temperatures are mild rather than hot, typically 18–24°C on good days.
Late spring (May) and early autumn (September) are quieter and often just as pleasant. The crowds thin, prices ease, and the landscape takes on good colour. These shoulder months work well for anyone primarily interested in the history and the museums.
Winter is quiet. The Viking Museum and Slottsfjellet tower remain open, but the waterfront loses its animation. Tønsberg is not a winter destination in the way that Oslo or Bergen are, though it is perfectly manageable as a short trip from Oslo at any time of year.
The Constitution Day celebrations on 17 May are worth catching if you happen to be in Vestfold: traditional processions and festivities fill the town centre.