"The three days and three nights seems to be a figure of speech. We do know that in Rabinnic thought any part of a day and a night was seen as the whole. They would use the term of a "day and a night" for any part of one. Notice we never hear about 40 days and 39 nights, or 3 days and 4 nights. It would seem quite strange that all the days rounded so nicely. If we do not understand it this way, then there really seems to be no way to reconcile it. Jesus was not burried for 3 24-hour periods. It is true that some put the crucifixion on a Wednesday. This is however a minority position among scholars and it still does not solve the issue. Wednesday plus three days still only brings us to Saturday. It seems best to understand Jesus' statement in light of the Rabinnic thought."
http://www.onenesspentecostal.com/threedays.htm