What is the greatest number of Friday the thirteenths that can occur in one year?

The greatest number of Friday the 13ths that can occur in one year is three. This happens when January 1st is a Sunday, leading to Friday the 13ths in January, April, and July. The specific alignment of the calendar allows this configuration, which can be verified by examining the day of the week for the 13th of each month in those years.