The first year
In the current calendar the years are counted starting from the birth of Jesus. The years following that date are named Anno Domini, abbreviated AD, while the years before Christ, are abbreviated as BC. The calculation was performed in the sixth century by the monk Dionysius the Little. He stated that Christ was born in the year 754 aUC, i.e. since the foundation of Rome. This year became the first of our era, what we now call 1 AD.
There is no “zero” year: 1 AD immediately follows 1 BC, because Zero as number was introduced later by the Arabs in Europe.
Recent studies demonstrate this date is not correct, because of the wrong setting of Christ's birth.
The king of the Jews, Herod the Great, who ordered the massacre of the Innocents in order to kill even Jesus, died in 750 year from the foundation of Rome, that is 4 BC.
In addition to the Roman calendar which starts from 753 BC, the foundation of Rome, other calendars are using different counting as the Jewish calendar, which begins with the creation of the world in 3760 BC (calculated according to the Bible) and as the Islamic, starting from Egira date, ie 622 AD. The Russian counting is originated from Byzantine calendar and was used until 1700: it stated that the Creation occurred in 5508 BC and this was the first year. The change ordered by Tsar Peter the Great brought year 7208 of the Byzantine era to be 1700 AD.
The first day of the year
The Julian and Gregorian calendars were commonly used, but the first day of the year changed considerably among different countries and cities. In ancient Rome we know the year began March 1. After Caesar’s reform the year began in January, so the last months added by Numa became the first ones. This costum was lost after the falling of the Roman Empire. The most frequent dates for New Year in Middle Ages were 25th March, Easter or 1st September. As a consequence counting the year was different from place to place depending when the year has started.
In England and Ireland until 1752 the New Year was celebrated on 25th March and also the Florentine republic followed the same date. In Spain it was on 25th December until early 1600, in France on Easter until 1564. The republic of Venice had New Year on 1st March and the Byzantines on 1st September. The Gregorian calendar reform served also to bring the New Year on January 1 and today this is the most followed one.
The rule for the calculation of Easter was fixed during the Council of Nicea in 325 AD. The Council of the Universal Christian Church, summoned by Emperor Constantine the Great, took place at Nicaea, in Bithynia, Asia Minor, to condemn the Arianism, a Christian sect, which was declared heresy. The council was the best chance to define also the calculation for Easter, which was celebrated on different days in the several Christian communities. The new rule stated: "All the Churches will celebrate Easter on the Sunday following the full moon after the vernal equinox." This date is based on the observation of the Sun and the Moon, while the Julian calendar was a solar-type calendar.
The astronomers in 325 AD indicated March 21 as the date of the vernal equinox. This date was fixed as the vernal equinox date, so to calculate the full moon after the fourteen days following the new moon. Actually we know that due to the precession of the equinoxes (see link) and the moon phase period of 7.2 days (more than seven days see above paragraph), these dates became are just two conventions and not astronomical phenomena.
Picture: Page from the lecture notes on astronomy compiled by the monk Magister Wolfgang de Styria in 1490 at Melk Abbey in Austria. At the lower right is a chart to calculate the date of Easter Sunday in the Julian calendar.
Source: Wikimedia Commons