In reply to PPP:
I used to reckon start of April if you pick your hills - lower ones , ones where access lies into the sun, westerly, etc, were reasonably safe as you get at that time of year though generally May was much better. Of course higher hills will still have snow even then and July is a better starting time!
Being flexible, prepare to abandon if necessary, avoiding obvious difficult ridges, etc, I've generally started early April over the years and had few that I've abandoned. Sometimes you have to alter your intended route.
This year though, could be a lot later and certainly will be for the higher hills.