Short answers, pulled from the story.
The word calendar originates from the Latin term calendar, which meant the first day of the month in the Roman calendar, derived from the verb to call out. This ancient verb referred to the public calling of the new moon when it was first seen in the sky, marking the beginning of a new month.
The Gregorian calendar was introduced in 1582 as a refinement to the Julian calendar. It corrected most of the remaining difference between the Julian calendar and the solar year, refining the system to better align with the Earth's orbit.
The Islamic calendar is based on the prohibition of intercalation by Muhammad, resulting in an observation-based lunar calendar that shifts relative to the seasons of the solar year. This decision was made in a sermon given on 9 Dhu al-Hijjah AH 10, which corresponds to the Julian date of the 6th of March 632.
The Roman calendar was reformed by Julius Caesar in 46 BC, creating the Julian calendar. This reform created a dissociation of the calendar month from lunation, shifting the focus from the moon to the sun and establishing a more predictable system for the Roman Empire.
The last European country to adopt the Gregorian calendar was Greece, in 1923. During the Early Modern period, its adoption was mostly limited to Roman Catholic nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade.