CREATE TABLE table_204_449 (id DECIMAL, "no" DECIMAL, "episode" TEXT, "title" TEXT, "original airdate" TEXT, "viewers" DECIMAL, "nightly\ rank" DECIMAL)

when did the last episode air ?
SELECT "original airdate" FROM table_204_449 ORDER BY "episode" DESC LIMIT 1
SELECT "original airdate" FROM "table_204_449" ORDER BY "episode" DESC NULLS LAST LIMIT 1