Beginning in late April most of the mountains start shutting down, in spite of the great snow-pack that still exists. Several ski areas will continue operations up to Memorial Day, although hours may be curtailed, or weekend-only. Squaw Valley has been most reliable about staying open late in the season. Alpine Meadows used to stay open, but didn't last year. Early May, I believe Heavenly was still open. Keep checking the web sites for closing dates and operating hours. They don't close for lack of snow, just lack of business.