The Gregorian calendar originated from the ancient Roman calendar. Roman English was originally only 10 months. The Roman Emperor decided to add two months to the end of the year. Later, Emperor Julius Caesar moved these two months to the beginning of the year, which became January. February, and January. The month becomes March. April, and so on. This is the Gregorian calendar in use in the world today.
In December-December, Roman Emperor Lucius wanted to name his mistress Amagonius for the last month of the year, but was opposed by the Senate. Therefore, the old name Decem is still used in December, which means "10" in Latin. English December has evolved from this.