CREATE TABLE table_26550 ("No. in series" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" FLOAT)

When 21 is the number in series what is the air date?
SELECT "Original air date" FROM table_26550 WHERE "No. in series" = '21'
SELECT "Original air date" FROM "table_26550" WHERE "No. in series" = '21'