@Katin: Actually, 2 days. And Halo 3 had 2 days more.
Right now (after first week/2 days) Gears stands at 60% of what Halo 3 sold first week. It would be closer if Gears had 4 days but it wouldn't catch it. I said in other thread that Gears won't match Halo 3 first week (4 days) sales even with 2 weeks (9 days) of sales. 50% drop is natural for games in UK and Gears 2 will be lucky to get that kind of drop.
@papflesje: Most games are released on Friday, just like Gears 2 and have 2 days to sale in first week. There are exceptions: MGS had 3 days, Halo 3 4 days, GTA IV 5 days.