197 1 calendar and which calendar are exactly the same?

The calendars of 1982, 1993, 1999, 20 10 and 202 1 year are exactly the same as those of 197 1 year. According to experts, whether calendars separated by several years will be repeated depends on whether the number of days separated between two years is an integer multiple of 7.

Because it is exactly fifty years from 197 1 to the end of 2020, there are 13 leap years and 13 days more, which is exactly 365×50+ 13= 18263.

In fact, from a mathematical point of view, this coincidence is more like an interesting math problem.

If 65438+ 10/in a normal year is Monday, then 65438+3 1 in February in the same year is also Monday-because there are 365 days in a normal year and 364 days in 52 weeks, it is only the beginning of a new cycle after 52 7 days.

However, for leap years, because February has an extra day, if 65438+ October 1 is Monday, then 65438+ February 3 1 in the same year is Tuesday, and 65438+ October 1 in the next year is Wednesday, which is two days late.

According to this algorithm, in the first year after leap year, the calendar will be repeated every six years after five years; In the second and third years after leap year, the calendar will be repeated every 1 1 year after ten years. For leap years, it will be 28 years after 27 years before the calendar repeats.