SCHEMA:
CREATE TABLE table_name_56 (date VARCHAR, competition VARCHAR, venue VARCHAR)
USER:
What date has a competition of 2012 afc challenge cup, and dasarath rangasala stadium, kathmandu as the venue?

QUERY:
SELECT date FROM table_name_56 WHERE competition = "2012 afc challenge cup" AND venue = "dasarath rangasala stadium, kathmandu"