Why does the date of Easter change? Why does the date of Chinese New Year change? What about the date of Ramadan? Why do Orthodox Christians celebrate Christmas on a different day? Why does the tax year in the UK begin on April 6th?



Calendars are a fascinating attempt to unravel the movements of the sun and moon in order to tie them to the regulation of days on earth. The main problem is that there isn’t an exact number of lunations (the time from one new moon to the next) in a solar year. If there were 12 it would be easy but there are in fact 12.368 lunations to a solar year. So adjustments have to be made.

Here we shall look at 3 kinds of solution to this numerical mismatch.

Lunar calendars which are based on the cycle of the moon and ignore the seasons.

Solar calendars which are based on the movements of the earth in relation to the sun and ignore the cycles of the moon.

Lunisolar calendars which take into account both the sun and the moon.

Lunar Calendar

The lunar month runs from new moon to new moon. This is a period of 29½ days. So, 12 lunar months make a lunar year of 29.5 x 12 = 354 days.

To deal with the ½ day, the lunar calendar consists of 6 months of 29 days and 6 months of 30 days.

(6 x 29) + (6 x 30) = 174 + 180 = 354 days.

But this is 11 days short of the solar year of 365 days. So, the lunar calendar falls behind the solar calendar by 11 days each year and bears no relation to the seasons.

Islamic Prohibition of intercalary month
The prohibition of the intercalary month

The Islamic calendar is a pure lunar calendar which is only used for religious purposes. In fact, the addition of an extra month from time to time to even things up was expressly forbidden by the prophet Mohammed. This means that the fast of Ramadan begins 11 days earlier each year as do the other main festivals and holy days.

Solar Calendar

Solar calendars use the basic year of 365 days. They have 12 months which ignore the cycle of the moon. The month can begin at any time in the moon cycle and the new moon can fall on any day of the month.

Our civil calendar is a solar calendar. It begins on January 1st and stays in step with the seasons.
But as the solar year is in fact around 365.25 days, it was necessary to add an extra day to make a leap year of 366 days every four years in order to keep in step with the seasons. This is the Julian calendar that was proposed by Julius Caesar and introduced in 45 BCE.

Bust of Julius Caesar
Julius Caesar who proposed the Julian Calendar

But this still wasn’t close enough and the solar calendar was still slipping out of phase with the solstices and equinoxes and hence the seasons. So, the following amendment was introduced:

If a year is exactly divisible by 4 it is a leap year of 366 days.

But a century year that cannot be divided exactly by 400 – such as 1700, 1800, 1900 is a common year of 365 days.

But a century year that can be divided exactly by 400 – such as 1600 or 2000 is a leap year of 366 days


This calendar is known as the Gregorian Calendar after Pope Gregory XIII who introduced it in October 1582.


But as this calendar was introduced by the Pope it was only taken up by Catholic countries. Britain for example, as a Protestant country, only introduced it in 1752 when the difference between the Julian (Old style) and Gregorian (New style) calendars had mounted up to 11 days.

This loss of 11 days didn’t bother most people except for accountants for whom it represented a problem. Before the calendar reform, the year began on March 25th (Lady Day otherwise known as the Feast of the Annunciation) which works out as 9 months before 25th December when the nativity of Jesus Christ is celebrated. Lady Day was also the beginning of the tax year, so to keep the year the same length for accounting purposes, the beginning of the tax year was advanced by the 11 missing days to start on April 6th. Thus it still remains in the UK.

But Britain wasn’t the only country to lag behind the rest of the world in the implementation of the Gregorian calendar. Russia didn’t introduce it till after the revolution in 1918, Turkey only in 1926 and Saudi Arabia not till 2016. And the Eastern Orthodox Churches still use a liturgical calendar based on the Julian Calendar which means that they celebrate Christmas Day on January 6th for example.

Lunisolar Calendar

Another solution to the difference between 12 lunar months and one solar year is to add an extra ‘intercalary’ month. This is how it works:

12 lunar months of 29.5 days give a lunar year of 354 days.
A solar year is 365 days.
The difference is 365 – 354 = 11 days.

So, if we add an extra month every 2 or 3 years, we keep the lunar calendar in step with the solar year. This means that some years will have 12 months and others will have 13.

The Chinese calendar works like this, so does the Jewish calendar.

For example, the Chinese (Lunar) New Year is the second new moon after the Winter solstice on December 21st. From an astrological point of view, this will be the new moon in Aquarius; in other words any time from January 20th to February 18th.

In the Jewish calendar, the festival of Passover (Pesach) takes place on the first full moon after the Spring Equinox of March 21st, unless it’s a year with an extra month in which case it takes place the full moon after.

But it was important for Christians to keep the connection between the Jewish Passover and the Last Supper. So, for Catholics and Protestants, Easter is timed to be celebrated the weekend after Passover – in other words the Sunday after the full moon after the Spring Equinox.

More information on the date of Easter can be found here.

Ben Belinsky

Thanks to Bev for the question that sparked this off

29th February 2020

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