It happens at a set time in relation to official sunset each night, so it changes every night as official sunset time changes every day. I had the Lighting Designer tell me but for the life of me can't remember what it is and no cast members seem to know the magic formula.
I wanna say it was like 15 or 20 min after official sunset time while it is still dusk but don't hold me to that.
Best place to watch is smack dab in the middle of Cars land in the middle of the street. The sequence starts down at the entrance of Cars Land and lights up shop by shop and makes it way down to RSR.