Solar cycle (calendar)


The solar cycle is a 28-year cycle of the Julian calendar, and 400-year cycle of the Gregorian calendar with respect to the week. It occurs because leap years occur every 4 years and there are 7 possible days to start a leap year, making a 28-year sequence.
This cycle also occurs in the Gregorian calendar, but it is interrupted by years such as 1700, 1800, 1900, 2100, 2200, 2300 and 2500, which are divisible by four but which are common years. This interruption has the effect of skipping 16 years of the solar cycle between February 28 and March 1. Because the Gregorian cycle of 400 years has exactly 146,097 days, i.e. exactly 20,871 weeks, one can say that the Gregorian so-called solar cycle lasts 400 years.
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.
Unless a year is not a leap year due to Gregorian exceptions, a sequence of calendars is reused every 28 years.
The name solar cycle comes from Sunday, the traditional first day of the week.

Examples

Julian calendar

[Dominical letter]

The dominical letter in the Julian calendar depends only on the remainder of the year mod 28.
Year mod 28Dominical letter
0DC
1B
2A
3G
4FE
5D
6C
7B
8AG
9F
10E
11D
12CB
13A
14G
15F
16ED
17C
18B
19A
20GF
21E
22D
23C
24BA
25G
26F
27E

Doomsday">Doomsday rule">Doomsday

Friday the 13ths

Gregorian calendar

The years 1700, 1800, and 1900 were not leap years and do not fit in the tables of Dominical letters below.

Doomsday">Doomsday rule">Doomsday

First days of months

1583–1699

1701–1799

1801–1899

1901–2099