CREATE TABLE table_38598 ("Date" TEXT, "Tournament" TEXT, "Surface" TEXT, "Opponent in the final" TEXT, "Score" TEXT)

What surface was used on may 13, 2007?
SELECT "Surface" FROM table_38598 WHERE "Date" = 'may 13, 2007'
SELECT "Surface" FROM "table_38598" WHERE "Date" = 'may 13, 2007'