I never turn a light on before they calve, all it does is run everyone out of the barn, they don't seem to like lights on in their bedroom any more than we do.  After they have calved yes leave a light on so the calf can see what he is doing.

5th set of twin this year cow had her second lifetime set, had the first one fine, second was found dead with sac over pretty much whole body, she licked about 1/3 of a back leg.  Guessing she got up and had the first calf running around and didn't bother with the second one.