Poole works in every season, but they are genuinely different holidays, and picking the wrong one for what you want is the usual mistake.
Spring (March to May)
Gardens come good, Compton Acres in particular, and the harbour shallows warm faster than the open sea. Beaches are quiet, prices are low, and the walking is at its best before the summer haze. See Poole in spring.
Summer (June to August)
Peak everything: warmest air, longest days, the fullest events calendar, and the busiest beaches and roads. Book accommodation months ahead and arrive at beach car parks early. See Poole in summer.
Autumn (September to November)
The regulars' season. September often has the warmest sea of the year with a fraction of August's crowds, and the light across the harbour is at its best. See Poole in autumn.
Winter (December to February)
Birds in numbers on the harbour mudflats, empty beaches, a Christmas calendar along the Quay and the lowest prices of the year. Expect reduced hours at some attractions. See Poole in winter.
For the detail on temperature, rain and what to pack, see weather.