What date did the episode air that had n/a for it's bbc three weekly ranking?
CREATE TABLE table_24399615_4 (
    airdate VARCHAR,
    bbc_three_weekly_ranking VARCHAR
)
SELECT airdate FROM table_24399615_4 WHERE bbc_three_weekly_ranking = "N/A"
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What date did the episode air that had n/a for it's bbc three weekly ranking? ### Input: CREATE TABLE table_24399615_4 (
    airdate VARCHAR,
    bbc_three_weekly_ranking VARCHAR
) ### Response: SELECT airdate FROM table_24399615_4 WHERE bbc_three_weekly_ranking = "N/A"