The abbreviation and few terms used in the below post has the following meanings:
IE or i.e. means the abbreviation for the Latin phrase id est, meaning “that is.”
solstices mean Solstices are a planet’s longest and shortest days of the year.
The time taken for the earth to orbit the sun is not an exact multiple of solar days. It’s a bit under 365.25 solar days, but in general use, we take it 365 days in a year but in a leap year which comes after every four years has 366 days per year, The year which is completely divisible by 4 is a leap year and it will have 366 days.for example, 2000,2004,2008,2012,2016 and 2020 are leap year. but the extra. 25 part of the day will become one whole day after 4 years. for example( 0.25 x 4=1 ) this one day is a result of a leap year. all the solstices and equinoxes, stay at the same place in the calendar. That was the Julian calendar. However as I said the solar year is a bit under 365.25 solar days, so by the middle ages the equinoxes and solstices had drifted by about 10 days, this was a problem for fixing the date of Easter. Pope Gregory assembled some mathematicians to return up with an answer. They came up with a leap year every four years unless the year was divisible by 100 in which case there would not be a leap year unless it was divisible by 400. Thus 1900 wasn’t an intercalary year but 2000 was. This was introduced to the catholic world in 1582 and was gradually accepted by the rest over the next few hundred years. The only place that I know of that still uses the Julian calendar is the monastic community of Mount Athos. The Gregorian calendar has an accumulative error of about 26 seconds per annum.
What other options are there?
Well, there are actually several.
The Hebrew calendar: A year has 12 lunar months, IE each month starts with a new moon, this is about 10 or 11 days shorter than a solar year. Roughly every three years an extra month is inserted. Originally this was done by observation which makes dating Biblical events difficult, now it’s done by calculation. there is a benefit to this, as Passover must occur at a full moon the date of the crucifixion can be calculated. I and several others came up with April 33.
The Chinese calendar. This is lunar with each month starting with a new moon. The New Year is fixed because of the second new phase of the moon after the solstice hemisphere. The Chinese year has no fixed length.
The French Revolutionary calendar: Every year started with the Autumnal equinox hemisphere as timed in Paris. Each month had 30 days, three ‘decades’ of ten days each. This leaves 5 or 6 days after the end of Fructidor, the 12th month. These days were the Jours Complémentaire AKA the Sansculottides. Originally this was observational, but later the Romme convention made it mathematical. It was officially abandoned 1 January 1806.
The Hijri calendar: This is a lunar calendar with 12 months, each starting with a new moon. This makes it 10 or 11 days shorter than the solar year. It cycles through the solar year. There is no correlation between a month and a season. Today is 12 June 2019 it’s also 9 Shawwal 1440 AH in about 15 years Shawwal will roughly correspond with December and in about 30 years it’ll be about June again.