user: Question: What is the date of appointment when the date of vacancy is 21 December 2007?
Columns: ['Team', 'Outgoing manager', 'Manner of departure', 'Date of vacancy', 'Replaced by', 'Date of appointment', 'Position in table']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Manchester City', 'Stuart Pearce', 'Contract terminated', '14 May 2007', 'Sven-Göran Eriksson', '6 July 2007', 'Pre-season']
SQL:

assistant: SELECT "Date of appointment" FROM "Table" WHERE "Date of vacancy" = '21 December 2007';