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_16965 ( "Series no." real, "Title" text, "Directed by" text, "Written by" text, "Original air date" text, "Production Code" real )

### Question ###
What was the airdate of 21 series number?

### Accurate SQL ###

SELECT "Original air date" FROM table_16965 WHERE "Series no." = '21'