user: Question: Who was the vacator when the date successor seated was August 21, 1973?
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Michigan 7th', 'Donald W. Riegle, Jr. (R)', 'Switched party affiliation', 'Donald W. Riegle, Jr. (D)', 'February 27, 1973']
SQL:

assistant: SELECT "Vacator" FROM "Table" WHERE "Date successor seated" = 'August 21, 1973';