user: Question: What days are vacant that were replaced by John Brown?
Columns: ['Team', 'Outgoing manager', 'Manner of departure', 'Date of vacancy', 'Replaced by', 'Date of appointment']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Clyde', 'Joe Miller', 'Mutual consent', '25 May', 'Colin Hendry', '11 June']
SQL:

assistant: SELECT "Date of vacancy" FROM "Table" WHERE "Replaced by" = 'John Brown';