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first digit(6) is for number of lettes in 'Sunday', 7 is for continuing sequence and 13 is for number of letters + sequence starting with 1 (from Monday onwards)
The answer is
the first digit is the number of letters in the word "Sunday", the second digit is the position of Sunday in the week (starting from Monday), and the other digits are the sum of the first two.
This same pattern worked to generate all the other numbers for Monday to Saturday - check it!
The first digit is the number of letters in the day. The second digit is 0 because you start counting the days of the week with Sunday, not Monday, and you start counting at zero. The last digit is the sum of the first two.
Depends if you count Sunday as the start of the week or not.
If you do its 0606
If not then its 6713