Have you ever wondered why Chinese New Year falls on a different date each year? Or why Christmas is always on the same date but Easter changes? Ramadan is on a different date every year, too. Well, it depends on which calendar you're using.
The calendar now in general use worldwide, for secular purposes at least, is called the Gregorian Calendar (we’ll discuss the exceptions of Iran, Afghanistan, Nepal and Ethiopia further down). It is a solar dating system proclaimed by Pope Gregory XIII in 1582. It was based on the Julian calendar established by Julius Caesar, on the advice of Alexandrian astronomer Sosigenes, who introduced the Egyptian solar calendar.
This system divided the year into 12 months which all had 30 or 31 days except for February, which had 28 days, and 29 days in every fourth year. The Julian measurement calculated the solar year as 365.25 days, however it is actually 365 days, 5 hours, 48 minutes, 45.25 seconds.
By Pope Gregory’s time, the date did not match the season, so he advanced the date by 10 days. Also, in the Gregorian calendar, century years cannot be leap years unless they are exactly divisible by 400 (such as 1600 and 2000).
The Egyptian exception
The Ancient Egyptian Calendar, unlike its contemporaries that focussed on the Moon, was based on the Sun and was the predecessor to the Julian Calendar. It had 365 days and 12 months, each consisting of three, 10-day weeks. The last five days of the year were the birthdays of five deities. There were three seasons, however the calendar got further and further away from correlating with the seasons because they did not have a leap year.
The Coptic Calendar
The Egyptian exception leads us to the Coptic Calendar. When Pope Gregory implemented the Gregorian Calendar in 1582, Christians who were not aligned with the Roman Catholic Church decided to stick with the Julian calendar (at least initially). The Coptic Orthodox Church, a Christian church founded in Egypt by the Apostle Mark, has its own calendar based on the Ancient Egyptian system.
According to the Coptic calendar, the first day of the year is 11 September, or 12 September in leap years and Christmas – the birth of Jesus – is celebrated on 7 January. Easter – when Christians believe Jesus rose from the dead – is celebrated on the same day as some Orthodox Easters (determined by the Julian calendar). In contemporary Egypt, Easter Monday is a public holiday and it also coincides with Sham El Nessim, a national festival marking the start of Spring that dates back to ancient times.
The Ethiopian Calendar
The Ethiopian Calendar, also known as the Ge’ez or Amharic Calendar, is based on the ancient Coptic Calendar.
It also has 12, 30-day months plus five or six days, sometimes known as the thirteenth month, to match the Sun's cycles. Like the Coptic Calendar, Ethiopian New Year, called Enkutatash, is celebrated on 11 or 12 September according to the Gregorian Calendar. Another big celebration for Ethiopian Orthodox Christians is Meskel, celebrated on 27 or 28 September to commemorate the Finding of the True Cross, on which Jesus was believed to be crucified.
Ge’ez is the ancient language of northern Ethiopia and southern Eritrea. Amharic, Afan Oromo, Afar, Somali and Tigrigna are the official languages of contemporary Ethiopia (the later four were added in 2020).
Nepal is another country that does not use the Gregorian Calendar. The official calendar is called Bikram Sambat or Vikram Samvat. It’s a lunar Hindu calendar used in Nepal and some Indian states.
There’s also Nepal Sambat, another lunar calendar used to determine the dates of religious festivals, birthdays and death anniversaries. One of the most important festivals in Nepal is Dashain, which generally happens from late September to mid-October at the end of the monsoon season.
Chinese New Year
Another significant date celebrated in many countries around the world is Chinese New Year, or Lunar New Year. The Gregorian calendar is used in China for secular purposes, however the traditional lunisolar Chinese calendar determines important festival dates such as Lunar New Year.
Lunisolar calendars use the Moon to calculate months and the Sun to calculate years. This method was also used historically in the Middle East, except Egypt, and Ancient Greece.
Read more about Chinese New Year here and here.
The Islamic Hijri Calendar
The world’s approximately 1.8 billion people of Muslim faith use the Islamic Calendar, also known as the Hijri calendar, to determine their key religious dates.
It is designed to follow the Moon's cycles and contains 12 lunar months that begin when a new moon is sighted and each year has 354 or 355 days. Each month alternates between 29 and 30 days, except the last month, which varies based on a 30-year cycle.
The Hijri Year begins on the day the Prophet Muhammad started his migration from Mecca to Medina to escape persecution. The holy month of Ramadan, when Muslims fast from sunrise to sunset to deepen their relationship with Allah, is the ninth month in the Hijri calendar.
Ramadan ends with the feast of Eid Al-Fitr. Another major feast day in the Islamic calendar is Eid al-Adha, which marks the end of the hajj, the holy pilgrimage to Mecca that Muslims who are financially and physically capable must complete at least once in their lives.
The Solar Hijri Calendar
In Iran and Afghanistan, the official calendar is the Solar Hijri calendar, also known as the Iranian or Persian Calendar. It is based on astronomical observations of the Sun and is the most accurate calendar in the world.
Like the Islamic Hijri calendar, the year count for the Solar Hijri calendar also began with the Prophet Mohammed’s migration to Medina in 622 CE. However, because the Islamic Calendar is lunar and the Persian Calendar is solar, they are now up to different years.
Why is Easter on a different date every year?
The date of Easter is based on the lunar cycle. It falls on the first Sunday after the first full Moon following the Northern Hemisphere Spring equinox, which falls on 20 or 21 March.
Go with the seasons
Seasons play an important part in determining key dates in calendars across many cultures. For example, the traditional Persian New Year, called Nourooz, dating back to the pre-Islamic religion of Zoroastrianism, falls on the first day of the Northern Hemisphere Spring.
Similarly, Chinese New Year marks the "Start of Spring’ and falls between 21 January and 20 February on the Gregorian calendar.
So how are the seasons defined? Countries further away from the Equator have four seasons defined as:
Spring, which begins when the Sun is exactly above the Equator, so night and day are equal lengths (20 or 21 March in the Northern Hemisphere and 22 or 23 September in the Southern Hemisphere). This is called an equinox and occurs twice a year.
Summer, which begins on the day of the year with the longest daylight hours (20 or 21 June in the Northern Hemisphere and 21 or 22 December in the Southern Hemisphere).
Autumn also begins on an equinox (20 or 21 September in the Northern Hemisphere and 22 or 23 March in the Southern Hemisphere).
Winter begins on the shortest day of the year with the longest night (20 or 21 December in the Northern Hemisphere and 21 or 22 June in the Southern Hemisphere).
In Australia, the Bureau of Meteorology starts the seasons on the first of the month (Spring begins on 1 September, Summer on 1 December, Autumn on 1 March and Winter on 1 June). This is just for simplicity.