They have their schedule here.
But just in case your finger is tired from clicking...
Lifts turn from 9 a.m. to 9 p.m. on night ski nights. Keystone is open 9 a.m.- 4 p.m. non-Night Skiing evenings. Night skiing and riding is available on Dercum Mountain and in the A51 Terrain Park on the following days during the 07/08 season:
Wednesday - Sunday, 11/21 - 11/25The terrain park features set on the Spring Dipper run for Early Season jibbing and riding will close at 4 p.m. Night jibbing will be available when A51 opens later in the season.
Friday - Saturday 11/30 - 12/15
7 days a week 12/19 - 1/6
Wednesday - Sunday 1/9 - 2/24
7 days a week 2/27 - 3/30