This is a discussion i have regularly with clients when looking at external work during the year, some clients insist that they have the outside work done in the summer, and some are happy to leave it with you, as long as its before the harsh weather sets in.
Typically i would start to consider my external work around May -Oct... This year in particularly has dispelled the myth that you cant work in the early months. March saw some of the loveliest weather for external work with warm weeks and a medium temperature for working in. The summer had its weeks of blaring sunshine which can be both uncomfortable for the decorator and awful for drying times speeding up, yet i still prefer to do the bulk of my externals in Aug - Oct as the temp cools and the weather is still fair...
I do have to convince many that October is still a decent time for working outside, but i can guarantee that Indian summers are pretty regular and with some overcast days its a very comfortable scenario to work, all i would consider is that any masonry work using Weathershield or Sandtex is brought forward to when the days are dry, although some products have increased drying times to help prolong the working months outside for the decorator.