Easter had shifted so far away from the vernal equinox that Pope Gregory XIII ordered the calendar's adjustment, resulting in the Gregorian calendar. History Prehistoric lunar calendar The original Roman calendar is believed to have been an observational lunar calendar whose months began from the first signs of a new crescent moon. Because a lunar cycle is about days long, such months would have varied between . Twelve such months would have fallen short of the solar year; without adjustment, such a year would have quickly rotated out of alignment with the seasons in the manner of the Islamic calendar. Given the seasonal aspects of the later calendar and its associated religious festivals, this was presumably avoided through some form of intercalation or the suspension of the calendar during winter. Rome's 8-day week, the nundinal cycle, was shared with the Etruscans, who used it as the schedule of royal audiences. It was presumably a part of the early calendar and was credited in Roman legend variously to Romulus and Servius Tullius. Legendary 10 month calendar The Romans themselves described their first organized year as one with ten fixed months, each of . Such a decimal division fitted general Roman practice. The four 31 day months (March, May, Quintilis, and October) were called "full" () and the others "hollow" (). Its 304 days made up exactly 38 nundinal cycles. The system is usually said to have left the remaining 50 odd days of the year as an unorganized "winter", although Licinius Macer's lost history apparently stated the earliest Roman calendar employed intercalation instead and Macrobius claims the 10 month calendar was allowed to shift until the summer and winter months were completely misplaced, at which time additional days belonging to no month were simply inserted into the calendar until it seemed things were restored to their proper place. Later Roman writers credited this calendar to Romulus, their legendary first king and culture hero, although this was common with other practices and traditions whose origin had been lost to them. Some scholars doubt the existence of this calendar at all, as it is only attested in late Republican and Imperial sources and supported only by the misplaced names of the months from September to December. Rüpke also finds the coincidence of the length of the supposed "Romulan" year with the length of the first ten months of the Julian calendar to be