2月为啥只有28天？竟然与凯撒和屋大维有关！李永乐老师讲公历演化（2018最新）

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People have known for a long time from astronomical observations that the time of year is a little over 365 days.
Because they are not integers, we need to make up the difference by setting up leap years or leap months.
In ancient Rome, there were ten months in a year, 355 days in a year, and 377 in leap years.
Because leap years were set by civil servants, the ancient people who had the poor communication is often not timely known, resulting in the whole society to date confusion.

Julius Caesar, the last ruler of the Roman republic, was determined to change the situation and followed the advice of astronomers, creating the Julian calendar.
The Julian calendar divides a year into 12 months, 31 days for the big month, 30 days for the small month, and 29 days for February. Every four years is a leap year, and February is 30days in leap years, which is the beginning of the modern Gregorian calendar.

In 1582, Pope Gregory the 13th decreed a ten-day deletion
It also stipulateed the leap year rule of "one leap in every four years, no leap in every a hundred years, and one leap again in every four hundred years ", which is called the Gregorian calendar.