The Gregorian calendar is the most prevalently used calendar today. Within this calendar, a standard year consists of 365 days with a leap day being introduced to the month of February during a leap year. The months of April, June, September, and November have 30 days, while the rest have 31 days except for February, which has 28 days in a standard year, and 29 in a leap year.
The Gregorian calendar is a reformed version of the Julian calendar, which was itself a modification of the ancient Roman calendar. The ancient Roman calendar was believed to be an observational lunar calendar, based on the cycles of the moon's phases. The Romans were then believed to have adopted a 10-month calendar with 304 days, leaving the remaining 50 or so days as an unorganized winter. This calendar allowed the summer and winter months to become completely misplaced, leading to the adoption of more accurate calendars.
The Republican calendar later used by Rome followed Greek calendars in its assumptions of 29.5 days in a lunar cycle, and 12.5 synodic months in a solar year, which align every fourth year upon the addition of the intercalary months of January and February. From this point, many attempts were made to align the Republican calendar with the solar year including the addition of an extra month to certain years to supplant the lack of days in a particular year. In 46 BC, the calendar was further reformed by Julius Caesar, introducing an algorithm that removed the dependence of calendars from the observation of the new moon. In order to accomplish this, Caesar inserted an additional 10 days into the Republican calendar, making the total number of days in a year 365. He also added the intercalation of a leap day every fourth year, all in an attempt to further synchronize the Roman calendar with the solar year.