March to May
March to May. The spring shoulder, and the less reliable of the two shoulders — which is not how it is usually sold.
March is genuinely unsettled and can hand you a week of rain. April is better and often lovely, but it is showery, and Easter moves the crowds around dramatically: wherever it falls, that fortnight behaves like high summer in the cities and prices follow. May is the strongest of the three and, in the cities, close to the best month of the year — long days, warm air, gardens at their peak.
The catch is the water. The sea in May is cold enough that most people will not swim in it, and a lot of coastal infrastructure is only part-awake. Spring is for cities, countryside and low-altitude walking, not for the beach.