What does the tortoise on the calendar mean?

There were great events in ancient times. Tortoise shells are burned with fire, and their cracks are regarded as good or bad luck, so they are called turtles.

Supplementary content:

1. Turtle-making comes from "Li Zhouguan's Pure Supplement", which takes state affairs as the tortoise's eight lives. Zheng Xuan's Note: There are eight national issues that need turtle's decision to make a speech. In Li Zhouguan's Pure Supplement, Fan Guozhen, Bu Lijun, Bu Dafeng,

2. By burning the tortoise shell, it is believed that the shape of cracks can predict good or bad luck, and the incomplete basis obtained by studying and observing various signs can judge unknown things or predict the future.