That first warm sunny day of spring when the shorts come out for the first time... Usually in April, but 2012 was late March and 2013, 7th May. 17C in April sunshine feels so much warmer than 17C under August cloud or June rain. I hate, almost every year, mid Oct through mid Nov. Too late for heat, too early for cold and snow, very rarely any long anticyclonic spells- last year's 37 consecutive rain days in that period was a new low.