Make Your Own Calendar!

It's nice to be able to generate your own calendar for a particular year of your choosing, even if it is only to answer the question "On which day of the week did a particular date fall?". You might of course, wish to make your own calendar using pictures that you took with your camera - a personal calendar recording a year in the life of your family, friend or club.

You can use the program which you can download here, to make a calendar for any year in the range 1583 to 9999 inclusive. 1583 is when people started to use the Gregorian Calendar (see below) in earnest and 9999 - well who knows what will happen by then? (!!)

What you will see here

• Introduction of the Gregorian Calendar
• Practical Aspects of the Gregorian Calendar
• Downloading and setting up the calendar program
• Downloading and setting up the calendar program
• Using the calendar program
• Sources and Attributions
• "Please Read Me", including Disclaimers and Copyright

Introduction of the Gregorian Calendar

The Gregorian Calendar, which forms the basis of our present day date reckoning, kicked off in 1582. It was a modification, by Pope Gregory XIII, of the Julian Calendar, and more closely fitted the astronomical givens - namely the period of rotation of the earth around the sun. There is more about this in the references near the end of this page.

Once Pope Gregory had initiated this calendar, other parts of Europe and the world gradually adopted it. However, the introduction of the Gregorian Calendar by various parts of the world was nevertheless a slow process, as this table, adapted from the sources below, shows. Even within a country, adoption could be slow: hence the dates for the Dutch Republic and the Holy Roman Empire should strictly read from 1582 and 1583 respectively.

Practical Aspects of the Gregorian Calendar

The Gregorian Calendar is designed to match as closely as possible the time the earth takes to do a complete encirclement of the sun, while at the same allowing us to have complete days in any year. Having achieved these aspects to our satisfaction, after how many years does our calendar repeat?

Matching our Calendar to the Astronomical Givens

The Gregorian Calendar gives 365 days per year with an additional day every leap year. Leap Years are defined to be exactly divisible by four - except those years which are exactly divisible by 100 where the result is then not exactly further divisible by four! Thus, for example, in the Gregorian Calendar, 1600, 2000 and 2400 are leap years, but 1700, 1800, 1900, 2100, 2200, 2300 are not. All this ensures that our year has an average length of 365.2425 mean solar days (see Solar Time), which is a pretty good fit to actual astronomical observations! (Note that Sidereal Time refers to time-keeping by using the repeating pattern of the stars. Sidereal Time is very similar to, but not exactly the same as, Solar Time!!).

After How Many Years does our Calendar repeat?

In the Gregorian Calendar (which uses Solar Time!!), a given year will repeat at least after every 400 years, in terms of the following three aspects.

1. The day of the week of the first day of the year (1st January).
2. The day of the week of the last day of the year (31st December).
3. The number of days (365 or 366) in the year.
   (This number, of course, depends on the first and last days of that particular year.)

For example, in these three regards, in the Gregorian Calendar,

• 1621 and 2021 are equivalent
• 1600 and 2000 are equivalent
• 1701 and 2101 are equivalent
• 1599 and 1999 are equivalent

Why is this so?

• The "normal" Solar Year has 365 days or (364+1) days and 364 is exactly divisible by 7.
  
• Leap years add a further day, since each leap year has (364+1+1)days.
• Over 400 years, the Gregorian Calendar gives us (100-3) or 97 leap years.
• Hence, over 400 years, we have (400+97) or 497 extra days over and above (400 x 364) days.
• 497 is exactly divisible by 7 - i.e. no remainder.
• Quod Erat Demonstrandum or Q.E.D. for short!

For the years within the 400 year range considered above, does any given year, in terms of the day on which 1st January falls, the day on which 31st December falls and the associated number of days in the year, repeat after less than 400 years? In this case a "brute force" approach with a spread sheet will give an answer in the affirmative!

Between and including 1583 and 2383 we have the following numbers for normal and leap years with the start and end days of the week (1st January and 31st December) for each year.

Normal	number	Leap	number
Mon-Mon	86	Mon-Tue	26
Tue-Tue	88	Tue-Wed	28
Wed-Wed	86	Wed-Thu	28
Thu-Thu	88	Thu-Fri	26
Fri-Fri	86	Fri-Sat	30
Sat-Sat	87	Sat-Sun	26
Sun-Sun	86	Sun-Mon	30

These numbers total to 801 years as expected and the ratio of normal to leap years is roughly 3 to 1, again, as expected! Below, we see a detail of the pattern of the years between and including 1930 to 2070.

Downloading and Setting up the Calendar Program

Initial Aspects

Before proceeding, please bear the text at the link on Disclaimers, Copyright & "Readme" in mind. The executable files have ariginally been created to work from DOS (command-Prompt) mode on the 32-bit 80386 processor and have been tested in the environment described on the associated page. The executable files have also run successfully from DOS Command-Prompt) mode invoked from the 64-bit version of Windows10®.

English Language Version

"cal.exe" Calendar Generator v1.0. Left click on link to Download. This can be placed in, and run from, any directory of your choosing. Just click on the file to run. A Command Prompt Window will appear with relevant information. Enter the required year. The window will then close and return you to your computer desktop, and the output calendar file will appear - as a text file - in the same directory as "cal.exe". Everything nice and simple!

You can see the User Interface and a Sample Output File here.

German Language Version

"cal_de.exe" Calendar Generator v1.0 (Deutsch). Left click on link to Download. This is the German language version of the above, and can also be placed in, and run from, any directory. The output file will appear in the same directory. Everything nice and simple!

You can see the User Interface and a Sample Output File here.

Calendar Icon

"favicon_ca.ico" Icon. To download, right click on link and choose "save target as". This icon can be associated with the desk top and/or the task bar, as required.

Using the Calendar Program

Here we consider historical aptness and give a short mention of fonts that can be used to print out your calendar.

Historical Aptness

Bear in mind that the calendar you generate will be relevant for any particular part of the world only from the time the Gregorian Calendar was introduced in that part of the world. For example, for Britain it would be relevant from 1752 onwards, and for Portugal already from 1582.

Printing Out Your Calendar

The text file for your chosen year will fit on to a sheet of A4 paper. However, your file should of course be printed out using a Monospaced Font such as:

Consolas Courier Courier New

Fixedsys Lucida Console Terminal

I personally find "Lucida Console" quite attractive - the font that is! However, you'll find a more detailed list of Monospaced Typefaces at the link given here.

=== Have fun with the Calendar Program! ===

---

Sources and Attributions

Links for Historical Items

• http://en.wikipedia.org/wiki/Gregorian_calendar
• http://en.wikipedia.org/wiki/Solar_time
• http://www.infoplease.com/spot/gregorian1.html
• http://jonathan.rawle.org/hyperpedia/calendar.php
• http://www.timeanddate.com/calendar/gregorian-calendar.html

Attribution for Picture

On this web page, the picture of Pope Gregory XIII in the "history" section of this web page, is from the "Creative Commons®" initiative. This item is not being used here for commercial purposes, and therefore its use should be compliant with the terms of the Creative-Commons® license. If you, dear reader, have a legitimate legal reason as to why this should not be so, then please let me know via my "Please Read Me" page. In advance, Many Thanks!