user: Question:  what's the cover date where character(s) is X-Men ; Magneto
Columns: ['Character(s)', 'First Appearance', 'Cover Date', 'Publisher', 'Estimated Value']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Superman', 'Action Comics #1', 'June 1938', 'DC Comics', '$2,100,000']
SQL:

assistant: SELECT "Cover Date" FROM "Table" WHERE "Character(s)" = 'X-Men ; Magneto';