Butterfly Calendar

Butterfly Calendar weeks have seven days, but the two days of the weekend coincide with one date like the Weekend Rest Calendar.

Each month has exactly 25 dates numbered 1 to 25, which is 29 or 30 days with the weekends. A year is made of twelve and a half months.

Month	Name	Number of dates	Minimum days	Maximum days (weekend start)
1	Januar	25	29	30
2	Februar
3	Mar
4	Pupar
5	Tersar
6	Junar
7	Quintar
8	Sixar
9	Septar
10	Octar
11	Novar
12	Neccar
13	Farfar	13 (19)	15 (22)	16
Year		313 (319)	365 (372)	366

Note: sorry, there is no month called Caterpillar...

The thirteenth month is called Farfar and it has 13 dates, which is 15 or 16 days with the weekends.

A month always ends on the same week day it started with, except when it starts on a weekend (Saturday) and ends on a weekend (Sunday) which adds a day to make it a 30-day month (or 16-day half month in Farfar). The same applies to a year.

Leap week

In the Butterfly Calendar a year with 366 days occurs every six years when the year starts with a weekend. That gives an average year length of 365.16667 days.

To prevent the calendar from drifting too far from Earth seasons, a leap week is added to Farfar every 93 years. That is called a farfal year. The introduction of farfal years gives an average year length of 365.24194 days.

On a farfal year the month of Farfar has 19 instead of 13 dates, which is 22 days with the weekends.

A farfal year always starts on a Monday or a Thursday and ends on the same week day. A farfal year has exactly 319 dates or 372 days.

The farfal year usually occurs once in a century. Last farfal year before today was 1953 and the next one will be 2046.

Dates

As mentioned earlier, the days of the month are numbered 1 to 25. That is not enough to tell the exact day since Saturday and Sunday share the same date.

For that reason, a date should always include the week day unless there is no need to differentiate Saturday from Sunday in the current context.

Years are counted from year 0000 in the proleptic Gregorian calendar, which was a farfal year and started/ended on a Thursday. That means all years multiple of six start on a Thursday.

Even-numbered years always start on a Tuesday, Thursday or weekend. Odd-numbered years always start on a Monday, Wednesday or Friday. Years end on the same they started (weekend = Saturday + Sunday).

Sample dates

Week day	Butterfly date	Gregorian date
Thursday	1 Jan 0000	30 Dec -0001	Start of farfal year
Monday	1 Jan 1953	29 Dec 1952	Start of farfal year
Monday	19 Far 1953	4 Jan 1954	End of year
Saturday	1 Jan 2000	1 Jan 2000
Sunday	13 Far 2000	31 Dec 2000	End of year
Monday	22 Ter 2024	20 May 2024
Monday	1 Jan 2025	30 Dec 2024

Every 12 years, the six-year cycle for the 366-day Butterfly year aligns with the four-year cycle for leap years in the Gregorian calendar. Year 2000 is an example of that.

Calendar Month Layout

There are six possible layouts that each repeat twice a year. Farfar repeats Januar layout but with less dates (13 dates, shown in bold or 19 dates, shown in italics in a farfal year).

option 1 — Monday start

Mo	Tu	We	Th	Fr	SS
1	2	3	4	5	6
7	8	9	10	11	12
13	14	15	16	17	18
19	20	21	22	23	24
25

option 2 — Tuesday start

Mo	Tu	We	Th	Fr	SS
	1	2	3	4	5
6	7	8	9	10	11
12	13	14	15	16	17
18	19	20	21	22	23
24	25

option 3 — Wednesday start

Mo	Tu	We	Th	Fr	SS
		1	2	3	4
5	6	7	8	9	10
11	12	13	14	15	16
17	18	19	20	21	22
23	24	25

option 4 — Thursday start

Mo	Tu	We	Th	Fr	SS
			1	2	3
4	5	6	7	8	9
10	11	12	13	14	15
16	17	18	19	20	21
22	23	24	25

option 5 — Friday start

Mo	Tu	We	Th	Fr	SS
				1	2
3	4	5	6	7	8
9	10	11	12	13	14
15	16	17	18	19	20
21	22	23	24	25

option 6 — Weekend start (30 days month)

Mo	Tu	We	Th	Fr	SS
					1
2	3	4	5	6	7
8	9	10	11	12	13
14	15	16	17	18	19
20	21	22	23	24	25