CREATE TABLE table_2778 ("No." FLOAT, "#" FLOAT, "Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "U.S. viewers  ( million ) " CLOB, "Rank  ( week ) " FLOAT, "Original air date" CLOB)

What is the original air date of the episode that had a week ranking of 21?
SELECT "Original air date" FROM table_2778 WHERE "Rank (week)" = '21'
SELECT "Original air date" FROM "table_2778" WHERE "Rank (week)" = '21'