Ropp's Perpetual Calendar is for determining the day of the week for specific dates that might be of interest to the user. Covering only three centuries, it can hardly be said to be "perpetual," although that was the author's claim. After hours of work, the editor of The Struggler managed to convert the original, which covered 1700 to 2000, to what is presented here — 1896 to 2196. Instructions for using the calendar appear below.

To learn to use the calendar, let's use an example. Say your birthday is on May 18, and you want to know on what day of the week it will fall ten years hence, in 2011.

(1) First you find the column in which the year 2011 is listed. It is in the fifth column of the years section in the upper left part of the table, the gray section.

(2) Follow that column down to the row that says "May," in the blue section. Where the column and the row come together are the numbers 2, 7, and 5.

(3) Notice that the heading for that column is "19/20/21," meaning the 1900s, 2000s, and 2100s. Since you are interested in the year 2011, you will use. the number in the 2000s column, the middle number, which is 7. That is the calendar number in which you will find your answer. (If you wanted to know when your birthday occurred in 1911 and will occur in 2111, you would select the number 2 [for 1911] and 5 [for 2111].)

(4) So now, looking at the column headings in the right-hand section of the table, the green section, you go across to the column identified as "Cal. 7" (Calendar 7), and go down to the number 18 (for May 18). Beside it you will find a W, which stands for Wednesday. Your birthday in 2011 will be on a Wednesday. Using Calendar 2, you find that had you lived in 1911, your birthday would have been on a Friday. Using Calendar 5, you find that if you live until 2111, your birthday will be on a Monday. Whoop-dee-doo!

In leap years, use the yellow section instead of the blue section for January and February.

Calendar Differences

The perpetual calendar above deals only with the Gregorian system. There are many other kinds, including Aztec, Babylonian, Chinese, Egyptian, French republican, Greek, Hindu, Jewish, Julian, Mayan, Muslim, Roman republican, and Tibetan. Only two of these are of interest to most Orthodox Christian citizens of this country, the Gregorian and the Julian. As Americans they have to follow the Gregorian in their secular life, but as Orthodox Christians they should follow the Church calendar, which is most similar to the old Julian system. Let's examine the accuracy issue between the Julian and the Gregorian.

The Gregorian calendar year averages 365 days, 5 hours, 49 minutes, and 12 seconds. Though still too long by a number of seconds, it achieves this greater accuracy over the Julian calendar by skipping some of the so-called "leap years," which are 366 days long. In a leap year, February has 29 days instead of its usual 28. Generally a leap year occurs when a year's number is evenly divisible by 4, but, in the Gregorian system, years evenly divisible by 100, (and therefore also by 4) are not leap years unless they are also evenly divisible by 400.

The year 2000, for instance, was evenly divisible by 100 and by 400, and was therefore a leap year in both the Gregorian and the Julian. So will 2400, 2800, 3200, etc., be leap years in both systems. But the years 1700, 1800, and 1900 were not Gregorian leap years. Neither will 2100, 2200, 2300, 2500, 2600, 2700, etc., be leap years. Although they are evenly divisible by 100, they are not evenly divisible by 400. That makes them 365-day ("non-leap") years. This rule eliminates 3 leap years every 400 years, thereby achieving greater accuracy and increasing the discrepancy between the calendars by 3 days per four centuries, or an average of 0.75 days per century.

The Church calendar is quite similar to the Julian calendar. The Julian, introduced in Rome in 56 B.C., is 365 days and 6 hours long. It was used in England into the eighteenth century and in Russia into the twentieth century. The difference between the Church calendar and the Gregorian is that the Church calendar never simply eliminated ten days, as was done by the Gregorian when it was first introduced, and does not make the additional correction of requiring centesimal years (those evenly divisible by 100) to be evenly divisible also by 400 to qualify as leap years. Therefore it has more leap years than the Gregorian, and this accounts for its greater average year length.

In the present century the difference between the Church and the Gregorian calendars is 13 days — the 10 that were dropped by the Gregorian, plus 3, because of the Church calendar having 3 leap years more than the Gregorian in the four centuries (1582 to 2001) since the introduction of the Gregorian.

How soon will the difference between the two calendars be 14 days? Well, we have to look for the next year that the Church calendar recognizes as a leap year but the Gregorian does not, i.e. in a year evenly divisible by 100 but not by 400. That has to be the year 2100.