February 20 17 12 is Saturday, so one day after the week, it must be Sunday, and seven days after the week, that is Saturday. Now do you understand that counting tails is not counting heads?
How many days before March 8?
29- 17+8=20 days, one day is three weeks, so March 8 is Thursday.
If from Monday, February 27th to March 8th, it is 10 days, and 10 equals a week +3 days, then Monday plus 3 days is Thursday. Still correctly judge the day and week.