Mostly the Best Odds Guarantee is available from 8 or 10am on the day of the race - however some online bookmakers will promote an early bird Best Odds Guaranteed available from the evening before the race. I would rely on Betfred for the earliest B.O.G. availability as they price their horse racing markets in the morning before the race and generally have the Best Odds Guaranteed feature available at the same time - in my experience anyway.