Why does Olympic soccer start before the opening ceremony?
In order to fit the soccer (or football in the UK) tournament into the 16 Olympic schedule they play qualifying rounds in the days before the opening ceremonies.
The first official day of Olympic soccer starts on July 28, 2012 with the women's first round groups E, F, and G. The women are scheduled to play July 31 and August 3 and 6. The medal events will be on August 9.
The men kick off July 29 with first round groups A, B, C, and D. They are scheduled to play August 1, 4, 7, and 10th with the medal events on August 11.