CREATE TABLE table_17031 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( millions ) " TEXT)

What is the original air date of season 18?
SELECT "Original air date" FROM table_17031 WHERE "No. in season" = '18'
SELECT "Original air date" FROM "table_17031" WHERE "No. in season" = '18'