The Gregorian calendar is today’s internationally accepted civil calendar and it’s also known as the “Western calendar” or “Christian calendar”. It was named after the man who first introduced it in February 1582: Pope Gregory XIII.

The calendar is strictly a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. 11 of the months have either 30 or 31 days, while the second month, February, has only 28 days during the common year.

Nearly every 4 years is a Leap Year, when one extra – or intercalary – day is added on 29 February.

Who designed the Gregorian calendar?

Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Italian doctor, astronomer and philosopher Luigi Lilio (also known as Aloysius Lilius). He was born around 1510 and died in 1576, six years before his calendar was officially introduced. Watch the video below to learn more.

Meanings of Day Names

The names of days of the week are from a combination of Roman and Germanic names for celestial bodies:

Sunday Latin “dies solis” => “Sun’s day”
Monday Latin “dies lunae” => “Moon’s day”
Tuesday Germanic “Tiw’s day” => “Mars’ day”
Wednesday Germanic “Woden’s day” => “Mercury’s day”
Thursday Germanic “Thor’s day” => “Jupiter’s day”
Friday Germanic “Frigg’s day” => “Venus’ day”
Saturday Latin “dies Saturni” => “Saturn’s day”

Meanings of Month Names

The names of the months are from the Roman calendar:

January Janus, protector of doorways
February Februalia, a time for sacrifice to atone for sins
March Mars, god of war
April Latin “aperire” => “to open” buds
May Maia, goddess of plant growth
June Latin “juvenis” => “youth”
July Julius Caesar
August Augustus Caesar
September Latin “septem” => “seven”
October Latin “octo” => “eight”
November Latin “novem” => “nine”
December Latin “decem” => “ten”

As you’ll notice, the last four months are numbered 7 to 10, which is an artifact of the time when the new year started in March.

Interesting Formulae

There’s another reason why the historical starting of the new year is significant. Starting with March, the length of months follows a mathematical series:

31 30 31 30 31 31 30 31 30 31 31 28

This means that you can calculate the day of week for any arbitrary day/month/year of the Gregorian calendar with the following formula (note all divisions are integral):

       _                                      _
      |     7 + 31*(m - 1)       y    y     y  |
dow = | d + -------------- + y + - - --- + --- | MOD 7
      |_          12             4   100   400_|
  

where

d := day of month (1..31)
m := month in old style (March = 1..February = 12)
y := year in old style
dow := day of week (Tuesday = 0..Monday = 6)

To convert from new style month/year to old style:

if (m > 2) m -= 2;             /* Mar-Dec: subtract 2 from month */
else m += 10,y–;             /* Jan-Feb: months 11 & 12 of previous year */

Here’s another fun formula. To find the number of days between two days, calculate a pair of calendar days with the formula (again, all divisions are integral), using new style month/year this time:

                      m
                  m + -
                      8              y    y     y
d + 30 * (m - 1) + ----- + y * 365 + - - --- + --- - ld
                     2               4   100   400

where:

d := day of month (1..31)
m := month in new style (January = 1..December = 12)
y := year in new style
ld := leap day correction factor:
0 for January and February in non-leap years
1 for January and February in leap years
2 for all other months in all years

In C code, the leap day correction factor is calculated as:

(m < 3) ? !(y % 4) && ((y % 100) || !(y % 400)) : 2

References: Wikipedia.com, unplugthetv.com, timeanddate.com & 12ghosts.com