user: Question: What was the date of appointment for Christos Kassianos who belonged to AEK?
Columns: ['Team', 'Outgoing manager', 'Manner of departure', 'Date of vacancy', 'Replaced by', 'Date of appointment']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Omonia', 'Giorgos Savvidis', 'Resigned', '4 March 2008', 'Nedim Tutić', '4 March 2008']
SQL:

assistant: SELECT "Date of appointment" FROM "Table" WHERE "Team" = 'AEK' AND "Replaced by" = 'Christos Kassianos';