Summer (December–February) is the obvious time to visit Hobart. Daytime temperatures sit around 20–22°C, the days are extremely long, and the city is alive with festivals — Taste of Tasmania around New Year, the finish of the Sydney–Hobart yacht race on 28 December, and the Wooden Boat Festival every two years in early February. The mountain is mostly snow-free and walkable.
The unusual choice is winter. Dark Mofo, MONA's winter solstice festival, has become the most distinctive cultural event in Australia: a fortnight of art installations, ritual fires, nude solstice swims, and serious music programming in mid-June. Cold, dark, expensive, unforgettable. Outside of festival time, Hobart winters are damp and grey, with snow on Mount Wellington and bracing winds off the Derwent — bring a coat.
Autumn (March–May) is underrated: clear skies, low rainfall, the Tarkine and inland forests turning gold, and far fewer crowds than summer.