CREATE TABLE table_16965 ("Series no." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production Code" FLOAT)

What was the airdate of 21 series number?
SELECT "Original air date" FROM table_16965 WHERE "Series no." = '21'
SELECT "Original air date" FROM "table_16965" WHERE "Series no." = '21'