In many places, month lengths immediately after that change were not fixed, but were based instead upon observation of the sky.
Priest-astronomers were assigned the duty of declaring when a new month began it was usually said to have started at the first sighting of a new moon.
The Babylonians, who lived in what is now Iraq, added an extra month to their years at irregular intervals.
Greek historian Diodorus Siculus wrote: "The approach to the Garden sloped like a hillside and the several parts of the structure rose from one another tier on tier... and was thickly planted with trees of every kind that, by their great size and other charm, gave pleasure to the beholder...
The water machines (raised) the water in great abundance from the river, although no one outside could see it." (Painting by Mario Larrinaga) The ancient Babylonians used a calendar with alternating 29- and 30-day months.
These additional days were considered to be very unlucky or unpropitious. Both of them observe three 365 day years followed by one 366 day year. It consists of twelve 30-day months with five "gatha days" added at the end of the year.
Two eastern Mediterranean peoples who did not embrace Islam were early Christians in upper Egypt, whom we now call Copts, and their neighbors to the south, the Ethiopians. Their years are divided into 12 months of 30 days each, and the extra five or six days are added after the twelfth month. Each of the thirty days as well as each of the gatha days has its own name.