TapionO58:

I didn't see anything about it taking place in 2535 but my guess is that it takes place on Reach. After all, Reach was the Largest and deadliest Covenant-Human battle before Halo. Plus the Spartans didn't have their MJOLNIR Battle Armor untill shortly before the Battle of Reach. It's tough to say. I first thought it was Harvest. Onyx takes place after Installation 04 is destroyed. It's descovered before Halo was descovered but the battle takes place after John destroys it. Plus, Onyx is eventually destroyed. It's hard to say. It could be any of them. If you think about it though, my best guess is that it's the Battle of Reach.

 They got the Mjolnir Mark IV in 2525... and thats the one in the x06 trailer the mark IV. But they didnt get the Mark V suit until 2552 so i dont think that the battle of reach is in the game.