If March 1, 2005 is Tuesday, and March 18 is the day of the week, I know the answer to this question, but I don't know why the 18 in formula 18 ／ 7 is If you know why, please answer me,

March 1, 2005 is Tuesday. What day is March 18?
Tuesday, March 1, after one day, is Wednesday, March 2;
After seven days, it's Tuesday, March 8th,
To March 18, after 18-1 = 17 days, 17 △ 7 = 2 Three, equivalent to three days, is Thursday