Probably because they were surrounded by Islamic peoples, Coptic and Ethiopian churches never adopted the Western calendar. They are referred to by that name just as we speak of a day by its number in the month.
Instead, these two isolated pockets of Christianity continued to use the old 360-day calendar. Beginning in 1906 of the Common Era, some modern Zoroastrians adopted the practice of adding an additional day every four years.
Month length at that time was simply the number of days that passed from one new lunar crescent to the next.
During those years in Rome, for example, a Pontifex (priest) observed the sky and announced a new moon and therefore the new month to the king.
This notion still exists today, months of 30 days in the Hebrew Calendar are called "full" and those with 29 are deemed to be "deficient." In addition to their declaring the beginning of each month based upon a sighting of the new moon, priest-astronomers were also charged with pinpointing the start of a year.
By observing the movement of Sirius, Egyptians came to grips with the fact that the year was more than five days longer than their venerable 360-day calendar.
This system required the addition of an extra month three times every eight years, and as a further adjustment the king would periodically order the insertion of an additional extra month into the calendar.
All of these peoples began their month when a young crescent was first seen in the sky.
This was probably because all months had previously been 30 days for such a long period of time.
During this period in Greece, for example, months that consisted of 30 days were considered to be "full;" those that lasted only 29 days were said to be "hollow." Months containing 30 days were also called "full" in Babylon, but those containing 29 were deemed to be "defective." After month lengths in the Celtic Calendar became fixed, those that had been given 30 days were termed "matos" (lucky) and those given 29 days "anmatos" (unlucky).
In order to account for these additional days, Egyptians created a myth about their sky-god, Nut.
During the reign of the Babylonian king Nabonasser (traditionally dated between 747 and 734 B. E.) priest/ astronomers in that country discontinued their practice of looking for the new moon in order to name the beginning of a month.In Persia under the Sassanids, and in Armenia and Cappadocia the official system of time-reckoning was twelve months of 30 days followed by five more days at the end of the year.