Year 0

Year zero does not exist in the Anno Domini system usually used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year 1 BC is followed by AD 1. However, there is a year zero in astronomical year numbering (where it coincides with the Julian year 1 BC) and in ISO 8601:2004 (where it coincides with the Gregorian year 1 BC) as well as in all Buddhist and Hindu calendars.

The Anno Domini era was introduced in 525 by Scythian monk Dionysius Exiguus (c. 470–c. 544), who used it to identify the years on his Easter table. He introduced the new era to avoid using the Diocletian era, based on the accession of Roman Emperor Diocletian, as he did not wish to continue the memory of a persecutor of Christians. In the preface to his Easter table, Dionysius stated that the "present year" was "the consulship of Probus Junior [Flavius Anicius Probus Iunior]" which was also 525 years "since the incarnation of our Lord Jesus Christ". How he arrived at that number is unknown.

Dionysius did not use AD years to date any historical event. This began with the English cleric Bede (c. 672–735), who used AD years in his Historia ecclesiastica gentis Anglorum (731), popularizing the era. Bede also used a term similar to the English before Christ once, but that practice did not catch on until very much later. Bede did not sequentially number days of the month, weeks of the year, or months of the year. However, he did number many of the days of the week using a counting origin of one in Ecclesiastical Latin. Previous Christian histories used anno mundi ("in the year of the world") beginning on the first day of creation, or anno Adami ("in the year of Adam") beginning at the creation of Adam five days later (the sixth day of creation according to the Genesis creation narrative), used by Africanus, or anno Abrahami ("in the year of Abraham") beginning 3,412 years after Creation according to the Septuagint, used by Eusebius of Caesarea, all of which assigned "one" to the year beginning at Creation, or the creation of Adam, or the birth of Abraham, respectively. Bede continued this earlier tradition relative to the AD era.

...
Wikipedia