There are seven days in a week. Suppose today is 1 day, 1+7=8, then the eighth day is also Friday. So 1+2 1=22, the 22nd day is Friday, and the 2nd1day is Thursday.

Another mathematical method: Let today be X, ask what day of the week is the y day from today, and there will be (x+y- 1)/7, and the remainder will be the day of the week. Take this as an example (5+2 1- 1)/7, and the remainder is 4, so it is Thursday.