Last trade day for ES MAR is Thursday of next week, March 18. So Monday the 15th would probably be a good roll date. Majority of volume should be in JUN by then.
I've been at it for 3 years and still have no idea exactly when.. but,
the day it switches .. the morning volume will be unusually low,
also if you're in touch with a chat room or other people posting charts, then it'll be obvious as well.