It is already September 19 in Japan, or August 15 in the old Japanese-Chinese calendar. This day is the Harvest Moon festival in East Asian culture. On this day, it is customary for the Japanese to spent the night viewing the moon and eating dango.
It's not directly stated, but DDC also contains enough signs to suggest today is the day DDC takes place. IN took place 9 years ago, during the Harvest Moon festival of 2004.
Let's celebrate the occasion, while Reimu and co. takes off to solve the incident.