It occurs because leap years occur every 4 years, typically observed by adding a day to the month of February, making it February 29th.
There are 7 possible days to start a leap year, making a 28-year sequence.
This interruption has the effect of skipping 16 years of the solar cycle between February 28 and March 1.
[2] Calendar years are usually marked by Dominical letters indicating the first Sunday in a new year, thus the term solar cycle can also refer to a repeating sequence of Dominical letters.
[3] Sun-based calendars are first thought to be used by the Egyptians, who based it around the annual sunrise of the Dog Star and flooding of the Nile River.