The fight will start at approx 4 am for Brits watching here

"The doors for the arena will open for spectators to make their way in at 2pm local time, with the first fight taking place an hour later. The main event, Mayweather vs Pacquiao, is currently listed for an 8pm local time start, but as always with these events, it could indeed be pushed later if the majority of fights on the undercard go the distance.
With regards to British time, the main event is set for a 4am start. Plenty of coffee and energy drinks at the ready if you are staying right through!"
What time does Mayweather vs Pacquiao start on Saturday? All you need to know about the fight - Mirror Online