In antiquity, the length of the year was well understood by the Romans and Greeks. Hipparchus estimated the length of the solar year to be 365 days, 5 hours and 55 minutes (which is just a little too long). The Julian calendar (named for Julius Caesar) assumed a year of 365¼ days - but the discrepancy was known almost 1500 years before it was fixed in the time of Pope Gregory. The Roman calendar was a 12 month lunar calendar, like the Jewish calendar, this drifted with the seasons, so from time to time they included 'intercalated' days and months. These were calculated by priests, but the calendar drifted as the priests either added in extra days due to politics, or they forgot to make the corrections. Adding extra days allowed favoured politicians to remain in office longer.
Romulus is supposed to have created the first calendar, with 10 months. This was dramatically out of sync with the seasons -but is why we have September, October, November and December (the seventh, eighth, ninth and tenth months). Other months were named for various Gods (e.g. Mars). The days were called kalends, and the month had special days called nones, March 7th, and Ides, march 15th.
So march 9th was called VII Ides, six days before Ides. March 13th, 2 days before Ides was III ides.
A 304 day calendar wasn't tremendously useful due to the variation of the seasons, so two extra months were added, Januarius and Februarius.
This gave 355 days in the year, which was better - but still flawed. They tried an extra month, but that made the year too long, then they added an extra month every eight years - which worked better, but still relied on the priests to remember to add in that extra month - and they forgot.
Caesar, returning from Egypt, found the system was in a mess - and ordered reform. To re-align the calendar, extra months were added and 46 BC lasted 445 days. Caesar also moved the start of the year to January instead of March (which is why the 10th month of October is named for the number 8). Caesar also ordered that the extra ten days be aded throughout the year. This gave a calendar with months of 30 or 31 days (alternating) - though February had 29 days. Thereafter we had the Julian calendar of 365 days with a leap year of 366 days every fourth year.
Later, the calendar was tinkered with again, with leap years coming every 3 years instead of 4, but this was corrected by Augustus (in whose honour Sextilis was renamed).
Annoyingly, they tweaked the lengths of the months for political reasons - August was beefed up to honour Augustus by stealing a day from February. Other changes were made which wrecked the neat pattern established in the length of the months. They also renamed other months to honour other Caesars - but these changes did not stick.
The Julian calendar continued in this way for a millenia and a half until Pope Gregory XIII made a reform in 1582, even then, the Julian calendar continued to be used in some countries who by then were not under the influence of the Papacy. The United Kingdom did not change until the 18th century, Sweden had a hybrid of the Julian Calendar and Gregorian reforms - they did not observe the modified Gregorian rule for not having a leap year if the century was not divisible by 400.
In the UK, Parliament were careful to specify how the calendar change of 11 days would affect contracts, birthdays, prison terms and so on. The same number of 'natural' days were to pass. Given that the year pre-reform started on the 25th March in the UK, this meant that taxes were not due until the 6th April. In the UK we still use this date as the start of a new tax year.
In the 4th century AD, the Roman empire was stagnating. Constantine acted to halt the decline by consolidating its structures - and as a result kept Rome going for another 100 years in the west. In the east it lasted another 1000 years.
A predecessor of Constantine had divided the Roman Empire into Eastern and Western halves. Constantine chose Byzantium (Constantinople) as his Capital. When Rome fell in the west, the Greek speaking Roman Empire would remain as the Byzantine Empire - which would persist until it fell to the Ottomans in 1453. The Byzantine Empire was a name given to it later, at the time the west referred to it as the Empire of the Greeks, and in the east, the Muslim world simply called it Rûm.
Constantine chose a new religion of the east to secure his political holding, and it wasn't clear which one. There were many popular sects, and a military victory at the time was attributed to several Gods.
Later, the story was that Constantine is said to have had a dream that he would be assured victory in battle if he marched under the banner of Christ. It was said that due to this, Constantine embraced Christianity, however, the story came after the decision - Christianity was more as an act of politics than anything else.
Christianity eventually won out, and it affected the Roman Calendar. Sunday was named as the holy day, which changed the precedent previously set in both Judaism and Roman paganism that Saturday would be the day of worship. The reason for this was that Christ is crucified on the sixth day of the Jewish week, and rose on the first day of the following week - a Sunday. As the calendar spread, some day names from existing cultures 'stuck'. So in English we have Woden's Day (Wednesday), Woden being the god of poetry. This replaced Mercurius' day - named for the Roman God of communication. Friday is Freyja's or Frigg's day - a West Germanic translation of dies Veneris, or Venus's day (Vendredi in French)
Choosing Sunday as the holy day, dies Solis, also pleased the various sun worshippers. This must have annoyed the bishops who made the best of it by proclaiming that Christ was lighting up the world.
Easter was, and still is, a big deal for Christians. It's the defining act of Christianity. The trouble is that it is based on the Jewish calendar, a Lunar calendar - and the Roman world was adopting a Solar calendar. The two drift against each other.
It didn't matter to Constantine when Easter was celebrated, as long as there was agreement - it was part of bringing the Empire together.
Constantine held a council at Nicaea (Νίκαια), which was Greek for 'Victory'. Constantine was keen for the meeting to be attended by as many Bishops as possible, and he paid for their travel and lodgings along the road. The purpose of the meeting was to settle questions about Christianity. Wikipedia lists the agenda thus:
- The Arian question regarding the relationship between God the Father and Jesus; i.e. are the Father and Son one in purpose only or also one in being;
- The date of celebration of the Paschal/Easter observation
- The Meletian schism;
- The validity of baptism by heretics;
- The status of the lapsed in the persecution under Licinius.
Christians had celebrated Easter according to the Jewish lunar calendar. Matthew, Mark and Luke place the event on the Sunday after the passover feast, whereas John indicates a different Sunday (straight away, a literal interpretation of the Bible is not a defensible position).
The early Christians had different interpretations - a major embarrassment for the most holy of occasions, add to this the issue of anti-Semitism which was arising, and it caused problems for early Christians to base their holy day on a Jewish calendar and decisions about the calendar by the Jewish priests.
The decision was made to link easter to the solar year, and base it upon the Vernal equinox. It was decided that Easter would be the first sunday on the first full moon after the equinox - but it shall never be on the Jewish festival of Passover.
This was problematic, as establishing this date was not easy - the date of the vernal equinox was not easy to establish (for practical purposes March 21st was used, but the equinox does drift). In addition the lunar calendar drifts against the solar calendar - the dates of the full moons are not the same each year.
Add into this the fact that the Julian Calendar of 365¼ days was 11 minutes out, and over time, the calculation of the 'true' date of Easter was a real problem for Christianity. During the dark ages in Europe, it was this problem that kept scientific enquiry on life-support - whereas it flourished in the east.
By the time of Pope Gregory, the 11 minutes error had built up significantly, the equinox was on the 14th March, and using the 21st could give large errors in the 'true' date of Easter. Add to this some political problems, where in Rome the 25th March was used as the equinox, but in the east the 21st was used. There were also differences in how the lunar cycle was computed. This lead to an east-west split in the church which persists to this day.
It was the issue of celebrating easter on the wrong day that lead Pope Gregory XIII to issue his bull making the correction - a decree made after the church had begun to fragment in western europe - which meant that it took almost 200 years for the calendars to move back into synchronisation - though several countries did not adopt the Gregorian calendar until the 20th Century.
This isn't the end of the story. There are various reforms of the Gregorian calendar proposed. The tinkering with the length of months in Roman times means that the year doesn't neatly divide into quarters. The first quarter has 90 days, the second has 91, the third has 92, the fourth has 92. This means that financial comparisons between quarters are not valid.
One such reform is the 'World Calendar' This has 30 days in January, 31 in February, 30 in March. The pattern of 30/31/30 repeats over the year with Jan 1st always being a Sunday. This gives 364 days. The 365th day is called a 'Worlds Day' and is not a named day of the week - it would be between December 30th and January 1st.
In leap years, there would be a Worlds Day between June and July.
This calendar was close to adoption in the 20th century, but failed due to the USA not following through on the proposals. There are also religious objections as due to the intercalary days, the holy day would not be every seven days.
So, for the foreseeable future, our Calendar with it's foibles, due to its contorted history of religion and politics looks set to stay - I think it's doubtful that the World Calendar will be adopted in 2012 (as the World Calendar Association proposes).