The schedule of swimming events at the 2008 Summer Olympics occurred in two segments:
For the pool events, prelims were held in the evening, with semifinals/final in the following morning session, with a day between semifinals and finals in those events with semifinals. The shift of the normal morning preliminary heats and evening finals (to evening prelims and morning finals) occurred for the 2008 Olympics due to a request made by USA's broadcaster NBC, so that the finals from the event could be shown "live" in the USA. (Live ended up being 11:00 p.m.-midnight on the East Coast; with a tape delay to the same time for the West Coast.)
References
Swimming at the 2008 Summer Olympics – Event Order Wikipedia(Text) CC BY-SA