With these cold temps anytime of the day works well. I called one in on Sunday at 1pm. I then tried at dusk and under the full moonlight had one group hang up a couple hundred yards out and another set that wouldnt cross the river to get to me and I didnt have a clear shot at them. They are usually a little more skittish on public ground though.
As far as calling goes, I will start out with some kind of distress call, then if that don't work start going to lone howls, if that doesn't get a response then I will either go back to distress or a nasty sounding mt. Lion in heat sound that pretty much sounds like a cat backed up into a corner growling, then I will go straight back to group howls to get try to get them fired up and come in for the attack. Suprisingly it has worked for me.
If I hear them howling though when I arrive to my spot, I usually try to get close as I can, then start out with coyote vocals.