Well, in my case, right around now! In the Yukon it's too chilly for me till spring really hits. I, like, Sean's wife, HATE riding in the cold and getting frozen ears. My first ride was two days ago and in the late afternoon it was around +2C out. That was cold enough for me as I wore a toque and still had cold ears!!
And it looks like most around here head out about the same time. Pretty much, when the highways (where we mostly ride) are clear of snow and sand, it's good to go!
Can't wait till we get the +20C days... ahhh...