Why is the Japanese calendar related to the five elements?

It's from China. The seven stars, the sun, the moon, fire, water, wood, gold and earth, are collectively called "Seven Obsidian". Ancient China, South Korea, Japan and North Korea used to call it seven days a week, such as "Sun Obsidian Day" is Sunday, "Moon Obsidian Day" is Monday, and so on.

In the ancient calendar of our country, the twenty-eight lodges are arranged in the order of day, month, fire, water, wood, gold and earth, and repeated every seven days, which is called "seven obsidians"; In the theory of seven evils, the sun is the head, the sun is the respect, and it is the beginning of all things. Nature should be put at the front, and the rest should be put in order.

Therefore, Japan usually uses the method of calculating the week from Japan to Japan, but in order to improve the calendar in China, we use Monday, Tuesday and Saturday. Because the seventh week is not easy to read, there are also Sunday in the west and Japan Day in Japan, so it is changed to Sunday.