For the Lakota people, Unhcegila — the Horned Serpent — was a feared creature that settled in the Black Hills. A Horned Serpent appears in multiple Native American mythologies, as well as other parts of the world.
The Mayans revered Kukulcán, the feathered serpent god, and the Temple of Kukulcán (El Castillo) at Chichen Itza is famous for its shadow drawing a serpent on the steps during March and September equinox.
Unhcegila appears every six weeks in the Horned Serpent calendar. These days are marked as sacred. They are counted separately from the days of the month. There are seventeen sacred days in the calendar:
| Sacred day | Gregorian date | Month | Notes |
|---|---|---|---|
| SL | 1 January* | January | leap year only |
| S1 | 22 January | January | |
| S2 | 12 February | February | 13 February on a leap year |
| S3 | 6 March | March | |
| S4 | 27 March | March | |
| S5 | 18 April | April | |
| S6 | 9 May | May | |
| S7 | 31 May | June | |
| S8 | 21 June | June | June solstice |
| S9 | 13 July | July | |
| S10 | 3 August | August | |
| S11 | 25 August | August | |
| S12 | 15 September | September | |
| S13 | 7 October | October | |
| S14 | 28 October | October | |
| S15 | 19 November | November | |
| S16 | 10 December | December |
Odd sacred days occur on odd years and even sacred days on even years.
The sacred day that falls on 1 January is special (SL) since it only occurs on leap years. That moves the next sacred day back by one day and S2 occurs on 13 instead of 12 February in the Gregorian calendar.
On non-leap years, only the night ending on the 1st of January is sacred if the year is even.
Months of the Year[]
February, March and April have 29 days. All other months have 30 days. Sacred days are not counted as days of the month.
Month names are the same as the Gregorian calendar.
| Leap year | Even year | Odd year | |
|---|---|---|---|
| January | 2 Jan | 1 Jan | 1 Jan |
| February | 1 Feb | 31 Jan | 1 Feb |
| March | 2 Mar | 2 Mar | 2 Mar |
| April | 1 Apr | 1 Apr | 1 Apr |
| May | 30 Apr | 30 Apr | 1 May |
| June | 31 May | 31 May | 1 Jun |
| July | 1 Jul | 1 Jul | 1 Jul |
| August | 31 Jul | 31 Jul | 1 Aug |
| September | 31 Aug | 31 Aug | 1 Sep |
| October | 1 Oct | 1 Oct | 1 Oct |
| November | 1 Nov | 1 Nov | 1 Nov |
| December | 1 Dec | 1 Dec | 2 Dec |
Days of the Week[]
Even though the calendar can be used with the traditional days of the week (Monday, ...) its layout is much simpler when the sacred days are considered outside the week cycle.
For that purpose we can define a new set of seven days that describe stages of life (from Navaho):
- ayo-oni (affection)
- naané (play)
- bihoosh (learn)
- chooni (a mate)
- yisté (sustenance)
- baolt (a teacher)
- han-aly (rest)
With that, the calendar is perpetual: a single page represents all the years. Explore it interactively on the calendar home page.
Horned Serpent perpetual calendar
Converting Dates[]
To convert from a Horned Serpent date to a Gregorian date:
- Sacred days: SL = 1 Jan, S1 = 22 Jan, S2 = leap -> 13 Feb, even -> 12 Feb, S3 = 6 Mar, S4 = 27 Mar, S5 = 18 Apr, S6 = 9 May, S7 = 31 May, S8 = 21 Jun, S9 = 13 Jul, S10 = 3 Aug, S11 = 25 Aug, S12 = 15 Sep, S13 = 7 Oct, S14 = 28 Oct, S15 = 19 Nov, S16 = 10 Dec.
- Day following a sacred day is always Ayo-oni, and each sacred day is one day of the week later than previous one. Use last sacred day (or Ayo 1 Jan) as anchor to find day of the week.
- For any HSC date, adjust by:
- Jan: leap -> +1 until 30 Jan, odd -> +1 from 22 Jan
- Feb: leap -> +1 from 13 Feb, even -> -1 until 12 Feb, 29 Feb -> 1 Mar
- Mar: leap/even -> +1 until 25 Mar, +2 after, odd -> +1 until 4 Mar, +2 after
- Apr: odd -> +1 from 18 Apr
- May: leap/even -> -1 until 9 May
- Jun: leap/even -> -1 until 21 Jun
- Jul: odd -> +1 from 13 Jul
- Aug: leap/even -> -1 until 3 Aug, odd -> +1 from 25 Aug
- Sep: leap/even -> -1 until 15 Sep
- Oct: leap/even -> +1 from 28 Oct, odd -> +1 from 7 Oct
- Nov: odd -> +1 from 19 Nov
- Dec: leap/even -> +1 from 10 Dec, odd -> +1 until 30 Dec
Sample Years[]
| January | February | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| SL | 1 | 2 | 3 | 4 | 5 | 6 | 1 | 2 | 3 | 4 | ||||
| 7 | 8 | 9 | 10 | 11 | 12 | 13 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | |
| 14 | 15 | 16 | 17 | 18 | 19 | 20 | 12 | S2 | 13 | 14 | 15 | 16 | 17 | |
| 21 | 22 | 23 | 24 | 25 | 26 | 27 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | |
| 28 | 29 | 30 | 25 | 26 | 27 | 28 | 29 | |||||||
| March | April | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | ||||||
| 3 | 4 | 5 | 6 | 7 | 8 | 9 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | |
| 17 | 18 | 19 | 20 | 21 | 22 | 23 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | |
| 24 | 25 | S4 | 26 | 27 | 28 | 29 | 29 | |||||||
| May | June | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 4 | 5 | 6 | 1 | 2 | 3 | ||||||
| 7 | 8 | 9 | S6 | 10 | 11 | 12 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | |
| 20 | 21 | 22 | 23 | 24 | 25 | 26 | 18 | 19 | 20 | 21 | S8 | 22 | 23 | |
| 27 | 28 | 29 | 30 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | ||||
| July | August | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 1 | 2 | 3 | S10 | 4 | |||
| 8 | 9 | 10 | 11 | 12 | 13 | 14 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | |
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | |
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | |
| 29 | 30 | 26 | 27 | 28 | 29 | 30 | ||||||||
| September | October | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 1 | 2 | 3 | 4 | 5 | 6 | |||||||
| 3 | 4 | 5 | 6 | 7 | 8 | 9 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | |
| 10 | 11 | 12 | 13 | 14 | 15 | S12 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | |
| 16 | 17 | 18 | 19 | 20 | 21 | 22 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | |
| 23 | 24 | 25 | 26 | 27 | 28 | 29 | S14 | 28 | 29 | 30 | ||||
| 30 | ||||||||||||||
| November | December | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 1 | |||||||||||
| 4 | 5 | 6 | 7 | 8 | 9 | 10 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | 9 | S16 | 10 | 11 | 12 | 13 | 14 | |
| 18 | 19 | 20 | 21 | 22 | 23 | 24 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | |
| 25 | 26 | 27 | 28 | 29 | 30 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | ||
| 29 | 30 | |||||||||||||
| January | February | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 4 | 5 | 1 | 2 | ||||||||
| 6 | 7 | 8 | 9 | 10 | 11 | 12 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | |
| 20 | 21 | S1 | 22 | 23 | 24 | 25 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | |
| 26 | 27 | 28 | 29 | 30 | 24 | 25 | 26 | 27 | 28 | 29 | ||||
| March | April | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | ||||||||
| 2 | 3 | 4 | S3 | 5 | 6 | 7 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | |
| 8 | 9 | 10 | 11 | 12 | 13 | 14 | 14 | 15 | 16 | 17 | S5 | 18 | 19 | |
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | |
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | 27 | 28 | 29 | |||||
| 29 | ||||||||||||||
| May | June | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 4 | S7 | 1 | |||||||||
| 5 | 6 | 7 | 8 | 9 | 10 | 11 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
| 12 | 13 | 14 | 15 | 16 | 17 | 18 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | |
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | |
| 26 | 27 | 28 | 29 | 30 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | |||
| 30 | ||||||||||||||
| July | August | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 4 | 5 | 6 | 1 | 2 | 3 | ||||||
| 7 | 8 | 9 | 10 | 11 | 12 | S9 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | |
| 20 | 21 | 22 | 23 | 24 | 25 | 26 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | |
| 27 | 28 | 29 | 30 | S11 | 25 | 26 | 27 | 28 | 29 | 30 | ||||
| September | October | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 1 | 2 | 3 | 4 | 5 | |||
| 8 | 9 | 10 | 11 | 12 | 13 | 14 | 6 | S13 | 7 | 8 | 9 | 10 | 11 | |
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | |
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | |
| 29 | 30 | 26 | 27 | 28 | 29 | 30 | ||||||||
| November | December | |||||||||||||
| Mon | Tue | Wed | Thu | Fri | Sat | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Sun | |
| 1 | 2 | 1 | 2 | 3 | 4 | 5 | 6 | |||||||
| 3 | 4 | 5 | 6 | 7 | 8 | 9 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | |
| 17 | 18 | S15 | 19 | 20 | 21 | 22 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | |
| 23 | 24 | 25 | 26 | 27 | 28 | 29 | 28 | 29 | 30 | |||||
| 30 | ||||||||||||||
How Does It Work?[]
The Horned Serpent calendar generally follows the Gregorian calendar except that it has a more regular structure. The calendar is based on a two-year cycle which is why even and odd years are different.
Unhcegila appears every six weeks
Looking at the perpetual calendar, even though there are six weeks in the interval between two sacred days (even or odd), that does not count the sacred day itself. The sacred day cycle is actually 43 days (6×7 days+1 day).
It so happens that 17×43 = 365+366 = 731. It means that 17 apparitions of Unhcegila correspond to the time of one normal year and one leap year.
Since leap years are always even in the Gregorian calendar, we put 9 sacred days in a leap year and 8 in an odd year. For even years which are not leap years we need to remove one day. That is why the leap sacred day SL only appears on leap years.
Now observe that 365 - 8 = 357 which is a multiple of 7 (same with 366 - 9), which means there is a round number of weeks in the year if you take out the sacred days. That is the basis of the perpetual calendar.
To finish off, 357 is three days short of 360 which would make all the months equal with 30 days. They are taken from February, March and April which are not close to the aphelion anyway. For a better synchronisation with the Gregorian calendar February could have 28 non-sacred days and March 30, but I find it more satisfying to assign them 29 days each.
That sets the form of the Horned Serpent calendar: twelve months of 30 days or 29 days for some, 51 weeks exactly per year with 17 sacred days distributed evenly throughout a two-year period (counted separately).