If the 13th is a Friday, then the 14th is a Saturday, and the 15th is a Sunday.  Subtracting 14 days (which is two weeks), we find that the first of the month is also a $\boxed{\text{Sunday}}$.
In any month with a Friday the Thirteenth, what day of the week is the first day of the same month?
{'answer': '\\text{Sunday}', 'subject': 'Number Theory', 'level': 3, 'unique_id': 'test/number_theory/1240.json'}