A common year starting on Friday is any non-leap year (i.e. a year with 365 days) that begins on Friday, 1 January, and ends on Friday, 31 December. Its dominical letter hence is C. The most recent year of such kind was 2021 and the next one will be 2027 in the Gregorian calendar,[1] or, likewise, 2022 and 2033 in the obsolete Julian calendar, see below for more. This common year is one of the three possible common years in which a century year can end on, and occurs in century years that yield a remainder of 100 when divided by 400. The most recent such year was 1700 and the next one will be 2100.
From July of the year that precedes this type of year until September in this type of year is the longest period (14 months) that occurs without a Friday the 17th. Leap years starting on Tuesday share this characteristic, from August of the common year that precedes it to October in that type of year, (e.g. 2007-08 and 2035-36). This type of year also has the longest period (also 14 months) without a Tuesday the 13th, from July of this year until September of the next common year (that being on Saturday), unless the next year is a leap year (which is also a Saturday), then the period is reduced to only 11 months (e.g. 1999-2000 and 2027-28).
This is the one of two types of years overall where a rectangular February is possible, in places where Monday is considered to be the first day of the week. Common years starting on Thursday share this characteristic, but only in places where Sunday is considered to be the first day of the week.
Additionally, this type of year has three months (February, March and November) beginning exactly on the first day of the week, in areas which Monday is considered the first day of the week. Leap years starting on Monday share this characteristic on the months of January, April and July.
Calendars
Calendar for any common year starting on Friday, presented as common in many English-speaking areas
January
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
February
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
March
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
April
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
May
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
June
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
July
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
August
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
September
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
October
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
November
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
December
Su
Mo
Tu
We
Th
Fr
Sa
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
ISO 8601-conformant calendar with week numbers for any common year starting on Friday (dominical letter C)
January
Wk
Mo
Tu
We
Th
Fr
Sa
Su
53
01
02
03
01
04
05
06
07
08
09
10
02
11
12
13
14
15
16
17
03
18
19
20
21
22
23
24
04
25
26
27
28
29
30
31
February
Wk
Mo
Tu
We
Th
Fr
Sa
Su
05
01
02
03
04
05
06
07
06
08
09
10
11
12
13
14
07
15
16
17
18
19
20
21
08
22
23
24
25
26
27
28
March
Wk
Mo
Tu
We
Th
Fr
Sa
Su
09
01
02
03
04
05
06
07
10
08
09
10
11
12
13
14
11
15
16
17
18
19
20
21
12
22
23
24
25
26
27
28
13
29
30
31
April
Wk
Mo
Tu
We
Th
Fr
Sa
Su
13
01
02
03
04
14
05
06
07
08
09
10
11
15
12
13
14
15
16
17
18
16
19
20
21
22
23
24
25
17
26
27
28
29
30
May
Wk
Mo
Tu
We
Th
Fr
Sa
Su
17
01
02
18
03
04
05
06
07
08
09
19
10
11
12
13
14
15
16
20
17
18
19
20
21
22
23
21
24
25
26
27
28
29
30
22
31
June
Wk
Mo
Tu
We
Th
Fr
Sa
Su
22
01
02
03
04
05
06
23
07
08
09
10
11
12
13
24
14
15
16
17
18
19
20
25
21
22
23
24
25
26
27
26
28
29
30
July
Wk
Mo
Tu
We
Th
Fr
Sa
Su
26
01
02
03
04
27
05
06
07
08
09
10
11
28
12
13
14
15
16
17
18
29
19
20
21
22
23
24
25
30
26
27
28
29
30
31
August
Wk
Mo
Tu
We
Th
Fr
Sa
Su
30
01
31
02
03
04
05
06
07
08
32
09
10
11
12
13
14
15
33
16
17
18
19
20
21
22
34
23
24
25
26
27
28
29
35
30
31
September
Wk
Mo
Tu
We
Th
Fr
Sa
Su
35
01
02
03
04
05
36
06
07
08
09
10
11
12
37
13
14
15
16
17
18
19
38
20
21
22
23
24
25
26
39
27
28
29
30
October
Wk
Mo
Tu
We
Th
Fr
Sa
Su
39
01
02
03
40
04
05
06
07
08
09
10
41
11
12
13
14
15
16
17
42
18
19
20
21
22
23
24
43
25
26
27
28
29
30
31
November
Wk
Mo
Tu
We
Th
Fr
Sa
Su
44
01
02
03
04
05
06
07
45
08
09
10
11
12
13
14
46
15
16
17
18
19
20
21
47
22
23
24
25
26
27
28
48
29
30
December
Wk
Mo
Tu
We
Th
Fr
Sa
Su
48
01
02
03
04
05
49
06
07
08
09
10
11
12
50
13
14
15
16
17
18
19
51
20
21
22
23
24
25
26
52
27
28
29
30
31
This is the only year type where the nth "Doomsday" (this year Sunday) is not in ISO week n; it is in ISO week n-1.
Applicable years
Gregorian calendar
In the (currently used) Gregorian calendar, alongside Sunday, Monday, Wednesday or Saturday, the fourteen types of year (seven common, seven leap) repeat in a 400-year cycle (20871 weeks). Forty-three common years per cycle or exactly 10.75% start on a Friday. The 28-year sub-cycle only spans across century years divisible by 400, e.g. 1600, 2000, and 2400.
For this kind of year, the ISO week 10 (which begins March 8) and all subsequent ISO weeks occur later than in all other years, and exactly one week later than Leap years starting on Thursday. Also, the ISO weeks in January and February occur later than all other common years, but leap years starting on Friday share this characteristic in January and February, until ISO week 8.
In the now-obsolete Julian calendar, the fourteen types of year (seven common, seven leap) repeat in a 28-year cycle (1461 weeks). This sequence occurs exactly once within a cycle, and every common letter thrice.
As the Julian calendar repeats after 28 years that means it will also repeat after 700 years, i.e. 25 cycles. The year's position in the cycle is given by the formula ((year + 8) mod 28) + 1). Years 4, 15 and 26 of the cycle are common years beginning on Friday. 2017 is year 10 of the cycle. Approximately 10.71% of all years are common years beginning on Friday.