If its time, its time. I wouldn't worry about the weather too much. I would be more concerned about the cows and letting them get some condition on before the very cold weather hits. The cows are hopefully entering their third trimester carrying calves and need to take care of the one inside and themselves more than a big calf outside.

Assuming these are spring calves about 7 months old, I would just put out some very good hay on the calves side of the fence, maybe add a single hot wire to or in front of a barb wire dividing fence at about calf nose height and go for it.

Good luck.

Jim at Dawn