CREATE TABLE table_9888 ("Team" TEXT, "Outgoing manager" TEXT, "Manner of departure" TEXT, "Date of vacancy" TEXT, "Replaced by" TEXT, "Date of appointment" TEXT)

What was the date of appointment for Christos Kassianos who belonged to AEK?
SELECT "Date of appointment" FROM table_9888 WHERE "Team" = 'aek' AND "Replaced by" = 'christos kassianos'
SELECT "Date of appointment" FROM "table_9888" WHERE "Replaced by" = 'christos kassianos' AND "Team" = 'aek'