Note that the months of Aergiuli (Foreyule) and Aeftergiuli (Afteryule) do not mean before and after Yule, but "Early Yule" and "Late Yule". This is because originally there was a "Yule Month", which was split between the early and late parts. The same is true of Aerlitha (Forelithe) and Aefterlitha (Afterlithe).
The epoch for the calendar is the year 444 C.E. and the era is called A.S.E. (Anglo-Saxon Era). The years are therefore approximately 444 years behind those of the Gregorian calendar.
Days of the week
The days of the week were named after Norse deities, apart from Saturday, Sunday and Monday, and have the same origins as the names that are still used today. The Old English names for the days of the week were: Sunnandæg, Monandæg, Tiwesdæg, Wodnesdæg, Þunresdæg, Frigedæg, and Sæternesdæg.
The main festive days observed in the calendar are as follows, with typical equivalent dates in the Gregorian calendar:
- Modraniht (Mothers' Night), 30 Aergiuli (21st December)
- Yule Day, 1 Aeftergiuli (22nd December)
- Yuletide, 1 - 12 Aeftergiuli (22nd December - 2nd January)
- Winter Cross Quarter, 15 Solmonath (3rd February)
- Ostara, 1 Eostremonath (20th - 21st March)
- Egg Moon (Movable - first full moon of spring, usually occurs in Eostremonath)
- Spring Cross Quarter, 16 Thrimilchi (5th May)
- Litha, 31 Aerlitha - 1 Aefterlitha (21st - 22nd June)
- Summer Cross Quarter, 16 Weodmonath (6th August)
- Harvest Moon (Movable - last full moon of summer)
- Mabon/Harvest Home, 31 Haligmonath (22nd - 23rd September)
- Autumn Cross Quarter, 16 Blotmonath (6th November)
In popular parlance, the Harvest Moon is the full moon closest to the autumnal equinox, therefore it could occur in the last half of Haligmonath or the first half of Winterfylleth. In more northern climes, however, harvest would tend to occur earlier than this, and this fits with it being the last full moon before the equinox in my calendar. This means that it will usually fall in the month of Haligmonath, which, interestingly enough, has the alternative name of "Harvestmonth" (Haerfestmonath).
More on leap years
The rule for leaps years is simple - if there are 366 days between one winter solstice and the next, then the year is a leap year. To calculate the date and time of the northern winter solstice I have used the method detailed in Jean Meeus's Astronomical Algorithms, 2nd Ed., Ch. 27, which gives results to an accuracy of within a minute for dates between 1,000 and 3,000 C.E.
Tracking the seasons
Basing a calendar on an astronomical event such as the winter solstice or the vernal equinox requires the ability to predict the future dates of these events, if it is desired to project the calendar into the future. The problem with this is that, due to the way the Earth spins (it has a "wobble" that means that the seasons move in relation to the background constellations but also, and probably more importantly, the apsides, which is the collective name for the perihelion, the closest point of the Earth's orbit to the sun, and the aphelion, the farthest point of the Earth's orbit to the sun, move in relation to the seasons), the seasons are different lengths and these lengths change over time. For example, the perihelion currently occurs in early January, but a few centuries ago it occurred at the time of the northern winter solstice, and in a few thousand years it will move into February and eventually occur around the time of the vernal equinox. This means that northern hemisphere winter is currently the shortest season, because the Earth moves faster in its orbit around perihelion, and it will get shorter until the perihelion reaches early February. Once it reaches the vernal equinox then winter and spring in the northern hemisphere will be equal in length, and summer and autumn will be longer, but also equal in length. Due to these changes, if the calendar is to be kept aligned to the seasons, adjustments will have to be made in the longer term.
As this is a complex subject, I will not go into further detail here, but the links below provide some very interesting reading, and graphs to show the long term trends in the seasons and how calendars can track them.
Calendar for 1575 (2018-19)