CREATE TABLE table_12722302_2 (original_air_date VARCHAR, us_viewers__million_ VARCHAR)

What was the original air date for the episode with 3.90 u.s. viewers (millions)?
SELECT original_air_date FROM table_12722302_2 WHERE us_viewers__million_ = "3.90"
SELECT "original_air_date" FROM "table_12722302_2" WHERE "3.90" = "us_viewers__million_"