Any time is a good time. In general, night time is best if legal, then early mornings! BUT, coyotes can be called any time of day. Look at TX., guys kill bunches of coyotes down there, a lot at night, BUT a lot during the day too and it's no colder there than where you are.
I go ever chance I get. The only weather that stops me is high winds, then I'll stay home. This year with all the snow and cold we're having, calling has been real good. BUT, like I said, it doesn't have to be cold to call coyotes! It helps because the cold makes the coyotes have to eat more and eat more often!