If you knew the mechanism of action you'd have a Nobel Prize! Nobody right now knows the mode of action with certainty, but there is a large body of evidence which indicates that these drugs work by signalling for new proteins to be made, for neurons to change how they are interacting, and possibly for new neurons to be made. All of these are processes that take days to weeks to gear up, which is probably why, even though the drugs have immediate biochemical effects, they don't tend to have immediate antidepressant effects.