Because November is still spring but closer to summer than September and October, summer starts on Dec 21, so temps are pretty nice. March and November are the best months, April is mainly OK but you can have some cold days, for example there were to days with 6C minimums in 2017 according to a web, October is similar, 20ish maximums but 10ish minimums many days. So actually March - November then April, October and perhaps February.