user: Question: On what date did the opponent in the Toshiaki Sakai final play on a Grass surface?
Columns: ['Date', 'Tournament', 'Surface', 'Opponent in the final', 'Score']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['15 August 2004', 'Islamabad', 'Clay', 'Toshiaki Sakai', '7–6(3) 7–6(5)']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Opponent in the final" = 'Toshiaki Sakai' AND "Surface" = 'Grass';