The answer is 6713.

For each day of the week, the number associated with it is composed as follows:

• The leftmost digit is the number of letters in the day of the week.
• The 2nd digit from the left is the day number of the week (Monday = 1, Tuesday = 2, ..., Sunday = 7)
• The remaining digits represent the sum of the first two digits. 6 + 1 = 7, 7 + 2 = 9, etc. In the case of Sunday, that's 6 + 7 = 13.