In a letter to House Speaker Paul Ryan sent on Wednesday, Treasury Secretary Steve Mnuchin "the outstanding debt of the United States will be at the statutory limit" at midnight on March 16. At that point, Treasury will have to utilize "extraordinary measures" for the federal government to keep paying its bills.
Mnuchin told Ryan at that point "Treasury will suspend the sale of State and Local Government Series (SLGS) securities" "until the debt limit is either raised or suspended."
Here's a copy of the full note from Mnuchin to Ryan:
Department of the Treasury