Common year

A common year is any year that has exactly 365 days, and so is not a leap year. For example, the years 2021, 2022, and 2023 are common years. Also, century years like 1800, 1900, 2100, 2200, and 2300 that are not divisible by 400 are common years in the Gregorian calendar. Those century years are also called exceptional common years.

Types of common years
There are 7 types of common years, one for each day of the week.

Starting on Sunday
For such a year, the dominical letter is A and Friday the 13ths occur on January 13 and October 13. Such years occur 43 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 2, 13, or 19 mod 28.

Starting on Monday
For such a year, the dominical letter is G and Friday the 13ths occur on April 13 and July 13. Such years occur 43 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 3, 14, or 25 mod 28.

Starting on Tuesday
For such a year, the dominical letter is F and Friday the 13ths occur on September 13 and December 13. Such years occur 44 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 9, 15, or 26 mod 28.

Starting on Wednesday
For such a year, the dominical letter is E and a Friday the 13th occurs on June 13. Such years occur 43 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 10, 21, or 27 mod 28.

Starting on Thursday
For such a year, the dominical letter is D and Friday the 13ths occur on February 13, March 13, and November 13. Such years occur 44 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 5, 11, or 22 mod 28.

Starting on Friday
For such a year, the dominical letter is C and a Friday the 13th occurs on August 13. Such years occur 43 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 6, 17, or 23 mod 28.

Starting on Saturday
For such a year, the dominical letter is B and a Friday the 13th occurs on May 13. Such years occur 43 times in the 400-year cycle of the Gregorian calendar and 3 times in the 28-year cycle of the Julian calendar.

In the Julian calendar, such years occur when the year reduces to 1, 7, or 18 mod 28.