Since the ecclesiastical calendar is based on lunar rather than solar cycles, certain key holidays (feasts) occur on different days each year. The method of calculating these feasts has also changed since the council of Nicea (325 A.D.). The button labeled "Calculate Holidays" calculates the dates of seven major feasts for the year entered in the field labeled "Year", and displays the results below this field. The only restrictions are that the number entered in the field "Year" must be an integer (no fractions) greater than zero. However, the holidays generated are only valid for dates since 325 A.D. (Early Christian and Roman dating is another story). Also, for purposes of calculation, I have assumed that the ecclesiastical year begins on January first, even though this standard was only gradually accepted. If you are working with early monastic documents you might want to consider that dates from December 25th through March may be "off" by one year. To a Benedictine, for instance (to whom the year began on December 25th), the feast of the Innocents in 1450 would be December 28th, 1450, while to others it might be December 28th, 1449. In fact, before 1582, most calendars did not have the year begin on January 1st, even though the calculation of the moveable feasts acted as if it did. In England, the year "began" either on December 25th, or, more frequently on March 25th (Lady Day), until 1752. These vagaries are not something I wanted to include in the calculations, since they often varied quite a bit. The calculations for the holidays will take into account the days dropped from the calendar when the "New style" was adopted, since these affect the month and day of Easter. Conventions about the beginning of the year are easily corrected for.
When the Pope Gregory revised the calendar in 1582, a certain number of days were omitted from the calendar at a particular time, resulting in two separate styles of dating. England persisted in using the "Old Style" until 1752, because of religious differences. The Old Style also often dated the beginning of the year from March 25 rather than January 1, and this area of the site takes this difference into account. Thus March 8, 1735, Old Style is really March 19, 1736 in the New Style! Occasionally, particularly in dating material between 1582 and 1755 or so, it becomes necessary to convert back and forth.
Here is a more detailed description of the transition year (in England): 1752.
The area called "Convert Old Style to New Style" converts the old style date to a new style date and returns an answer. The area called "Convert New Style to Old Style" converts the new style date to an old style date and enters it in the "Old style" fields. You must enter an integer greater than zero for the year.
You can also calculate the day of the week for both old and new style dates.
Remember that these forms assume that the year begins on March 25th. If you want to plug in dates derived from the "Ecclesiastical Holidays" area, be aware that you will have to subtract 1 from the year for all dates between January 1st and March 24th.
Don't try to use this site to calculate dates of documents between 1588 and 1752 because England is a special case during these years.