If I remember correctly, the USO is the only slam that plays the first round over 3 days, so the matches could be all over the place on Monday, Tuesday and Wednesday. You might want to check the 2004 USO thread in the archive and see if that gives you some idea of how they do the schedule.
Best twitter posts of the epic match:
@dougrobson - Whoever is supplying Mahut with hair gel needs to ink a sponsorship deal now.
@Wimbledon - FYI, with @Wimbledon live scoring devices, please add 50 to the games in the fifth set of the Isner v Mahut match.
@HolterMedia - Nice to see that a three-day test has come to tennis here @wimbledon.
I think it's usually the top half first unless the top seed asks for a later start (eg to allow recovery from an injury). I would guess if there's 3 days ( ) top half is across Mon-Tues and bottom half across Tues-Wed.