Hmmm....you know I was wondering. Lastnight we were in line for Buzz Lightyear and my son (age 2) was enamoured by the castle. He would point and say "Dad! Dad! Look the castle!" It was all lit up of course. I said "Oh it's pretty isn't it? Is that snow?" He said 'No. Not snow. It's lights." Pretty smart for 2, eh?
Anyway, I have to believe it's sometime around dusk - maybe 6pm. You might want to dial Disneyland Guest Relations and ask - or ask a Cast Member upon your next trip. It's really beautiful, isn't it?
They typically do it 3 times a night. 5:30, 7:30 and after the fireworks.
Typically, before the 7:30 and fireworks lighting, they turn the castle lights off about 15 minutes before the next lighting. The garland, hub tree and Christmas tree lights go off right after the lighting ceremony.
The castle lighting and snow happens at 5:30, 7:30, and following the "Believe in Holiday Magic" fireworks. Assuming you're talking about Christmas Eve, when the park closes at 9 and fireworks are scheduled for 8:45, that puts the castle lighting occurring at about 8:55 (I can never remember exactly how long "Believe" is).