There are vast differences across the calendars followed in various religions. But, most of the months are affected by the position of the moon. Number of days in a month of the Hellenic calendars, Hebrew Lunisolar calendar, and Islamic Lunar calendar starts from the sighting of the thin crescent of the new moon. Though, there are several kinds of calendars used around the world, they are actually a further classification of these three basic calendars- lunar calendar, solar calendar, and luni-solar/soli-lunar calendars. Today, we will discuss different types of calendars and months along with the number of days in each month.
1. Meaning of Month
A period of four weeks containing 28-31 days is considered a month. It is affected by the position of the moon. The time from any day in one month to the corresponding day of the next month is a period of four weeks or 30 days. (See How Many Months Have 28 Days?)
2. Lunar Calendar
This calendar usually has 12 synodic months i.e., the time interval in which the moon phases repeat themselves. In this calendar, there are 29 days in each month, and on average, there are 354 days in a year. Since the number of days in a month of the Islamic calendar is affected by the moon phases, hence it may range between 29 to 30 days. Though, most of these lunar calendars have become outdated.
3. Solar Calendar
The calendar which is based on the Earth’s revolution viz the number of days it takes for the Earth to revolve around the sun is considered the solar calendar. The Gregorian calendar which we follow is a solar calendar too. However, it has transformed from a lunisolar to a solar calendar. This follows the timing of the setting and rising of the sun based on the position of the Earth in its orbit.
4. Lunisolar Calendar
In this type of calendar, there are usually 12 synodic months but a thirteenth month is inserted once every few years. This extra month is added to keep the calendar in phase with the seasons. Two lunisolar calendars are the Hebrew calendar, and the Chinese calendar, which is mostly used in eastern Asia. (See How many different religions are there in the World?)
5. Roman Calendar
The ancient Roman calendar had only 10 months with 304 days. The months were from March to December. Later, two months were added to fill the winter gap, due to which a year consisted of 355 number of days in a month. In these twelve months, 4 months- March, May, July, and October had 31 days, the other 7 months-January, April, June, August, September, November, and December had 29 days while February had 28 days.
6. Hebrew Calendar
Also known as the Jewish calendar; it is a lunisolar calendar, used for religious observances in Israel. In this calendar, there are 12 months in a year with 354 days. Every 2-3 years, an extra month is added, which makes it up to 385 days that year. This extra month is called Adar 1 and occurs only 7 times in the span of 19 years. In regular years, another month named Adar 2 is considered and written in the calendar. (See How many months is 90 days?)
7. Islamic Calendar
This calendar is also known as Hijri Calendar. The number of days in a month may vary from 29-30 days, depending upon the crescent of the moon. When the new moon is sighted, the next day is considered the 1st day of the month. This can make the previous month be 28, 29, or 30 days. Hence, the number of days in a month is determined by the visibility of the moon, positioning of the earth, and sometimes, weather conditions also. (See What makes the moon look orange?)
8. Hindu Calendar
This calendar is a lunisolar calendar. A year, according to this calendar, has 354 days. But a total of 33 days is added to the calendar after every 3rd year, which creates an extra lunar month of 29 days.
9. Reformed Bengali Calendar
This calendar follows the solar months and is used in Bangladesh. According to this calendar, there are six seasons. This calendar is similar to the Gregorian Calendar in the total number of days and months. The difference is that, in a year, the first five months are of 31 days and the next seven months are of 30 days. While every 4 years, an extra day is added to the 11th month, making it 31 days. This 11th month in the Gregorian calendar is from mid-February to mid-March.
10. Baha’i Calendar
It is a solar calendar used by The Baha’i Faith. The calendar has 19 months and the number of days in a month is 19, making it 361 days in a year. Four extra days are added to the last two months to match the total number of days with the solar year. Although, during a leap year, another day is added. This extra period is known as Intercalary Day. Ultimately, the year has 365 days and during a leap year, it has 366 days. (Also see Why Do We Have Different Time Zones?)
11. Nanakshahi Calendar
The calendar is based on the composition, Barah Maha, which means 12 months, as mentioned by a Sikh Guru. It reflects the changes that are conveyed in nature during the period of twelve months. This calendar is followed by the people who follow Sikhism. In a year, the first five months are of 31 days and the next 6 months are of 30 days. The last month is of 30 days during a regular year and 31 days in a leap year, which occurs once every 4 years.
12. Iranian Calendar
This calendar is known to be the one of most accurate calendar systems in the world. It is currently being used in Afghanistan and Iran. This calendar year begins on the northern Spring equinox. It has 12 months in a year, with the first 6 months having 31 days and the next 5 months having 30 days. While the last month has 29 days in a regular year and 30 days during a leap year.
13. Kurdish Calendar
This calendar is used in the Kurdistan region of Iraq. The Islamic and Gregorian calendars are also used alongside. It is similar to the Iranian calendar, wherein it has 12 months in a year, in which there are 31 days in the first six months and 30 days in the next five months. In a regular year, it has 29 days in the last month and 30 days in the leap year. (See Why is Sunday the First Day of the Week?)
We have covered several calendars and the number of days in a month in each of those calendars, which is determined by the astronomical pattern or phenomenon they follow.