And this year's budget directly refers to that provision, which was included in the omnibus budget bill approved in December of 2011.
Lawmakers who want no change asked the Government Accountability Office to review the legal issues on 6-day delivery, and the GAO came down against the Postal Service.
"USPS asserts that it need not comply with the provision," wrote the GAO Legal Counsel Susan Polling.
"We disagree," the GAO wrote, saying the Postal Service is using a "faulty premise" for its legal argument.
You can read the GAO letter here.
Two Republicans meanwhile publicly urged the Postal Service to simply forget about the Congress and act like any real business would, and make needed financial changes.
"As proposed, the Postal Service is not eliminating a day of service, but is merely altering what products are delivered," wrote Sen. Tom Coburn (R-OK) and Rep. Darrell Issa (R-CA), which you can read here.
Technically, Coburn and Issa are correct; the Postal Service is not ending its work on Saturday, but won't deliver regular first class mail. Packages will still be delivered. Mail will still go to postal boxes.
"Without major, immediate restructuring actions, annual operating deficits will increase, and the Postal Service will sink much deeper into default on payments owed to taxpayers," the two Republicans argued.
It isn't clear how this will be resolved, as the Postal Service seems intent on making the Saturday mail change.