No. 1: 1+7- 1 = 7

Week 2:1+14-1=13

Week 3:1+21-1= 21

Week 4: 1+28- 1 = 28.

If it is Wednesday, the calculation is as follows:

3+7- 1=9

3+ 14- 1= 16

3+2 1- 1=23

3+28- 1=30

That is, the day of the week plus the number of weeks of the week *2 minus 1 is the date of this month. (If the first week of this month is less than one week, subtract 1 instead of 7.