Ultimately, you have to make a choice on what you think will happen in the current season, but a good starting point is to check a frost chart or a frost map and decide the threshold (just freezing, a medium freeze, or hard freeze). I tend to use medium freeze because we don't get that much precipitation as a rule, but it depends on where you live.

Even then I tend to wait a little longer to put them on, because once they're on, it's a long time until they come off. For me, midwest, it tends to mean they go on end of November and come off mid-March, but I often try to cheat until mid-December.