CREATE TABLE table_16786 ("Series #" FLOAT, "Episode title" TEXT, "Writer ( s ) " TEXT, "Director" TEXT, "U.S. viewers  ( millions ) " TEXT, "Original air date" TEXT)

What is the original air date when there were 12.81 million u.s viewers?
SELECT "Original air date" FROM table_16786 WHERE "U.S. viewers (millions)" = '12.81'
SELECT "Original air date" FROM "table_16786" WHERE "U.S. viewers (millions)" = '12.81'