In a letter to House Speaker Paul Ryan sent on Wednesday, Treasury Secretary Steve Mnuchin said "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" in order to prevent a default.
Here's a copy of the full note from Mnuchin to Ryan:
Government shutdownDepartment of the Treasury