If the 12th of a month is 4 days earlier than Monday, what day will be on the 25th of that month.
4 days earlier than Monday will be Thursday. So day 12 of that month is Thursday. Number of days between 25 and 12 is 13. Since 13 = 1 X 7 + 6, so excess days are 6. So we can count the day to Wednesday.