I believe Channel 4 decided to put it on a Thursday as the target audience will often got out on Fridays and thus it was decided to put it on a day earlier to try and get all us young 'uns to watch it. Though that was reported in one of the papers, so you can read into that what you want...
In all seriousness it simply boils down to ratings. Shortly after the start of Big Brother 7 ITV1 will be putting on Celebrity X Factor, and Channel 4 want to get people hooked on BB7 before they can get hooked on ITV's offereing by getting it in earlier.