In any country, the civil calendar is the calendar, or possibly one of several calendars, used within that country for civil, official or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations.
The most widespread civil calendar and de facto international standard is the Gregorian calendar. Though that calendar is associated with the Catholic Church and the papacy, it has been adopted, as a matter of convenience, by many secular and non-Christian countries. Some countries have retained the older Julian calendar, while others use other calendars.
Commonly, another calendar, or more than one, is used alongside the civil calendar. For example, Christian Churches have their own calendars, which they use to compute the dates for their own festivals, though most of these dates are then expressed relative to the civil calendar. In Christian terminology these festivals are called movable feasts. Very few Christian festivals are fixed in relation to the civil calendar, the most notable one being Christmas.
The same applies to Jews and to Muslims, who have their own calendars for religious purposes, and have dates for festivals that are fixed in their own calendars but change in the civil calendar dates.
Civil calendars worldwide
Most countries in the world use the Gregorian calendar as their sole civil calendar. Countries which don't use the Gregorian calendar are Saudi Arabia, which uses the Islamic calendar for all purposes, Ethiopia (Ethiopian calendar) and Iran and Afghanistan, which use the Persian calendar. Some countries use other calendars alongside the Gregorian calendar: Bangladesh (Bangla calendar), India (Indian national calendar) and Israel (Hebrew calendar). In Israel, the Gregorian calendar is called the "civil calendar" and "Gregorian calendar."