The world "Calendar" is an old roman term, it was used to designate the first of the month. We can also find the latin word "Calendrium" which means a register. This term inspired the french language since it adopted the term "Calendier", and from there
the english one "Calender".
A Calendar is a system that help people to organize days, for administrative, commercial or religious needs, a calendar is composed of different periods of time: days, weeks, months and years. A Calendar can also represented as a list of planned events such as a holidays calendar, or birthdays calendar.
The periods withing a calendar are associated with sun or moon cycles, the most modern one, is the lunisolar calendar, it means that the lunar calendar is synchronized with the solar calendar.