I welcome comments about this program. To forestall answering the same questions repeatedly, here are some notes about how I calculate the dates. If you have questions about how to use the applet, see the help file. I intend this calendar to have a certain degree of historical accuracy, but there are certain simplifications you should be aware of.

Moveable Feasts

Most moveable feasts are based on Easter. The Easter algorithm I use is an implementation of the formula devised by Zeller, which correctly generates Easter dates both before and after the Gregorian shift. After the Gregorian shift, Easter differs depending upon the locale you select. The "Eastern" locale uses the Orthodox method, which calculates Easter in the Julian calendar (as if the shift had never occurred). All other locales use the Western method, which not only takes the vernal equinox based on the Gregorian calendar, but also makes an adjustment to the lunar calendar used to determine the date of the full moon.

The calender modifications I've discussed so far only involve the civil calendar. Although today we think of the Gregorian reform as a correction of the solar (Julian) calendar, at least as important to those who enacted the reform was the correction to the lunar calendar used to track the Easter months. In attempts at calendar reform before the Gregorian, the lunar months were most frequently the target, probably because it was felt much easier to reform the part of the calendar only used by churchmen, rather than one deeply entrenched in civil life.
In 1582, Pope Gregory XIII decreed a modification to the Julian calendar. The reform consisted of a one-time correction in the date (skipping some days in the calendar) along with some tweaks to the rules of the calendar itself. For the civil calendar, the only substantive change was to omit 3 leap days every 400 years (in years evenly divisible by 100 but not 400, e.g., 1700, 1800, 1900, but not 2000). There were also changes to the way that the age of the moon was calculated for the purposes of finding the date of Easter.
Most historical calendars have been abstractions of the major observable astronomical cycles, especially the sun and moon. Widely different cultures often wind up with similar looking calendars, and at first sight it might seem as if direct influence were involved. While such may have been the case in certain instances, the conclusion is not necessary. If we consider the problem from the point of view of a hypothetical calendar designer, the possible calendars we can design are heavily determined both by the nature of the observed cycles and by the needs of our society. So what possibilities does our calendar designer have?
This program generates a Christian ecclesiastical calendar for any year you select. It will calculate the dates for the moveable feasts based on Easter (Ash Wednesday, Pentecost, etc.), Advent, and many saint and other feast days. The calendar filters holidays by date and locale.

To navigate in the calendar, you can click on the buttons beneath the grid display, which will scroll one unit at a time. Alternately, you can select the month by pulling down the choice box, or the year by clicking on the year box and editing it to the desired year (you need to hit the "enter" key for the year to change in the calendar grid). The calendar will correctly display BCE dates, but because of the year filtering, no holidays will appear in the list box.

The following article is adapted from a longer series of posts that I originally wrote for the Historical Writing section of Compuserve's Literary Forum in 1996. (The Literary Forum still exists under the name "Books and Writers Community", but its structure has radically changed, so most of the old sections are defunct.) I wrote the original in response to a skeptic (that's the polite word) who proposed the theory that conventional historians had grossly mistaken the chronology of both the ancient world and the Middle Ages. In the original piece, I tried to explain how historians piece together different facts to arrive at a chronology.


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