Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_23177573_1 (original_air_date VARCHAR, rank__week_ VARCHAR)

### Question ###
What is the original air date of the episode that had a week ranking of 21?

### Accurate SQL ###

SELECT original_air_date FROM table_23177573_1 WHERE rank__week_ = 21