Late in the season; November and December are preferable to me.
The water temps at that time of year are often higher than the air temp.
In the early Spring, the water is still so unbearably cold that the cold air blowing over the cold water goes right to your bones.
I have no problem sailing in cold fall conditions, but for some reason, the cold spring conditions get to me.
From last December on a friend's boat.
Of course sailing in these conditions bring on a whole new set of safety issues and concerns. First concern is not to fall in the water ever. Second concern is how not to fall in the water on an icy deck.