It's definitely at dusk, when you can't see the sun anymore, but it's not totally dark yet. Most of the lamposts will already be on though. Last year we were there the end of January. We were parked at the start of Rte. 66 watching the Pixar Parade at around 5:30. I was so distracted by the parade that when my husband kept nudging me I didn't pay attention and when I turned around I had just missed the neon coming on.
I will not miss it this year though! I will be there in March too and definitely want to see it and ride RSR at night! Dusk will come later though, especially since the time changes (Spring forward) on March 9th. Pay attention to what time dusk is for you a few days before you leave and then plan accordingly.
Have fun!!