No, he kept the names the ancient Romans gave them.
Calendar days are every day on the calendar, including weekend days, weekdays, business days and holidays. So, for example, 10 calendar days after the 4th of a month is the 14th of the same month.
The shortest "month" of the Gregorian calendar has 28 days.
The Mayans, Egyptians, Babylonians, and Chinese all had calendars in the ancient days. The calendar we use to today was first developed by the Romans, under Julius Caesar (the Julian Calendar). That was modified by the Gregorian Calendar (named for Pope Gregory XIII), which we still use today
December has 31 days in the Gregorian calendar, as that is the calendar that we now use.
Not in the Gregorian calendar.
The calendar that we use today is called the Gregorian calendar, created by Pope Gregory XIII. Before that, there was the Roman calendar, which was a lunar calendar, comprising 10 months and an intercalated month. Next came the Julian calendar, a twelve-month solar calendar introduced by Julius Caesar in 46 bc, consisting of 365 days, with an extra day every four years.
A twelve month lunar calendar is based on the cycles of the moon, with each month following a complete lunar cycle of approximately 29.5 days. This type of calendar typically has around 354-355 days in a year. In contrast, a twelve month solar calendar, like the Gregorian calendar, is based on the Earth's orbit around the sun, with each month roughly following the natural seasons. This calendar has 365 days in a common year and 366 days in a leap year.
From the first to the last day, inclusive, of any given month
The month of July is named after Julius Caesar and August is named after Caesar Augustus. They both changed the months and the amounts of days in them, giving us the calendar that our modern ones are based on.
The Islamic calendar month only has 28 days, where as the Georgian calendar month varies between 28 and 31 days (depending on the month). This means that the Islamic calendar is shorter than the Georgian calendar by approximately 11 days every year. For this reason Islamic religious evens such as the Holy month of Ramadan, migrate forward each year by about 11 days. Christian religious events tend to remain on the same day of the Georgian calendar each year (or within a few days).
The eighth month of a year of the Gregorian calendar, August, has 31 days.
Febuary