According to Norman mythology, millions of years ago, God created the world during the day the moon converged. Therefore, the Normans believed that God would descend upon them on that day and bless everyone.
The Single Moon Festival felt like Christmas or the Spring Festival on Earth, but the difference was that the moons did not converge on a regular basis, and would usually only happen every four or five years. The natives viewed the festival as very important, and was not an event that could be simply cancelled. Even the colonists, who set up various stores within the city, would use various discounts to earn a killing from the natives during this time.
New Washington had only known peace and prosperity. It was even well-known amongst the natives. If they actually cancelled festival celebrations, it would be a very big hit to various parties. After all, colonizing was not just about war; economy and profit were also things everyone was fighting for.