who came up with the modern day calendar
History of calendars The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582, and is today in worldwide use as the de facto calendar for secular purposes.
History of calendars The ancient Sumerian calendar divided a year into 12 lunar months of 29 or 30 days.[5] Each month began with the sighting of a new moon. Sumerian months had no uniform name throughout Sumer because of the religious diversity.[6] This resulted in scribes and scholars referring to them as "the first month", "the fifth month" etc.[citation needed] To keep the lunar year of 354 days in step with the solar year of 365.242 days an extra month was added periodically, much like a Gregorian leap year.[6] There were no weeks in the Sumerian calendar.[7] Holy days and time off from work were usually celebrated on the first, seventh and fifteenth of each month. In addition to these holy days, there were also feast days which varied from city to city.