During the Bronze Age, the priests living in Kosnia (the present Platinum Province as it was formerly known) divided a year into 12 months according to their astrology and farming calendars, starting the first calendar era for humans.
The Rain Moon was the fourth month of the 12 months. Nonstop rain enveloped most of the central and southern continent this month. Of course, to the North's Whiteland and Cold Plains, which were constantly enveloped in snow, and the West's Withering Desert, this month was only a name.
April Fool's Day on the 4th of the Rain Moon was a statutory festival recognized by most local officials. According to the officials, this wasn't a good time to deal with government affairs. It was a family holiday where one should stay at home to accompany their family and have a good time together.