If the 9th of a month is 6 days earlier than Monday, what day will be on the 28th of that month.
6 days earlier than Monday will be Tuesday. So day 9 of that month is Tuesday. Number of days between 28 and 9 is 19. Since 19 = 2 X 7 + 5, so excess days are 5. So we can count the day to Sunday.