A closer look at how this pronouncement came to be and the laws behind it suggest that there is a very good chance that Saturday delivery will continue beyond August.
In Washington speak; the Post Office has been funded by a continuing resolution that is passed annually. As part of this rider, Congress typically mandates that delivery must continue on Saturday. Because of the current budget mess in DC, the resolution is expiring in March. Therefore, the accompanying mandate that Saturday delivery service must be upheld is expiring as well.