CREATE TABLE table_57817 ("Date" CLOB, "Tournament" CLOB, "Surface" CLOB, "Opponent in the final" CLOB, "Score" CLOB)

On what date did the opponent in the Toshiaki Sakai final play on a grass surface?
SELECT "Date" FROM table_57817 WHERE "Opponent in the final" = 'toshiaki sakai' AND "Surface" = 'grass'
SELECT "Date" FROM "table_57817" WHERE "Opponent in the final" = 'toshiaki sakai' AND "Surface" = 'grass'