A simple, accurate, solar calender for Earth
Holidays and Holy Days
I have written this page from the perspective of someone living in the predominantly Christian country of Australia, where the most important holidays are Christmas and Easter. I realise that this is rather a biased perspective, so I apologise for that. The Earthian Calendar is meant for the whole Earth. When I have more time, perhaps I will add a section about Jewish, Islamic, Chinese, Buddhist holidays, etc. Either that, or I will happily append reader-contributed text on this topic, if provided (hint, hint!).
The date of December 25 came from the Roman festival called "Sol Invictus", at which several solar deities were worshipped. In those days December 25 was approximately the date of the winter solstice, although in modern times the winter solstice occurs on December 21 or 22.
It is unknown exactly when or why, but at some point this date became associated with Christ's birth. The earliest reference to celebrating the birth of Christ on December 25 was in 354 CE. However, it is actually extremely unlikely that Christ was born at this time of year, since few shepherds would be out tending their flocks by night in the middle of an Israeli winter.
A more plausible theory is that the Roman Catholic Church elected to hold "Christ's mass" (Christmas) at the time of the established Sol Invictus festival as a method of converting sun-worshipping pagans to Christianity.
In the Earthian Calendar, the northern winter solstice occurs on Capricornus 3 about 92% of the time, otherwise Capricornus 2 or 4. If an Earthian date is to be assigned to Christmas, then in my opinion Capricornus 3 would be best. This choice would bring the date of Christmas back to the winter solstice, which returns an astronomical basis to the date.
If, however, December 25 is to be retained for the sake of tradition, then Capricornus 6 may be preferred. Over the next 2000 years December 25 falls on Capricornus 6 about 57% of the time, Capricornus 7 about 41% of the time, and Capricornus 5 or 8 otherwise. Since Capricornus 6 occurs the most frequently, this is probably the best date to use. Boxing Day (December 26) would then be assigned to Capricornus 7, and Kepler's birthday (December 27) would be celebrated on Capricornus 8. (What?!? You don't celebrate Kepler's birthday?)
Computing the date of of Easter in the Christian calendar is rather complex (see Computus). It is defined as "the first Sunday after the first full moon on or after the vernal equinox", which in the Gregorian Calendar is always between March 22 and April 25 inclusive.
Since the dates and times of Sundays, full moons and vernal equinoxes are precisely the same in either calendar, Easter thus falls on precisely the same days.
Because the Earthian Calendar begins the year at the vernal equinox, Easter therefore almost always occurs in the first month of the year. The actual range of dates on which Easter Sunday can occur is from Aries 1 (New Year's Day) to Taurus 8 inclusive.