Table: tvb_series_2007_2
Columns: airing_date
english_title_chinese_title
number_of_episodes
genre
official_website

Question: What is the airing date for costume comedy that has 20 episodes?
SQL: SELECT "airing_date" FROM tvb_series_2007_2 WHERE "number_of_episodes" = 20 AND "genre" = 'costume comedy'