CREATE TABLE table_2579 ("No. in series" TEXT, "No. in season" TEXT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( millions ) " TEXT)

What day did the episode with a production code of 218 originally air?
SELECT "Original air date" FROM table_2579 WHERE "Production code" = '218'
SELECT "Original air date" FROM "table_2579" WHERE "Production code" = '218'