kalendars
A calendar is a system for arranging days into units such as years, months, and weeks, used to plan activities, schedule civil life, and mark festivals and historical anniversaries. The word calendar derives from Latin calendarium, meaning a book or register of accounts, and ultimately from calenda, the first day of the month in the old Roman market week.
There are several basic types: solar calendars, which count the solar year; lunar calendars, which count lunar
A calendar's structure typically includes years, months, weeks, and days. Months vary in length from 28 to
Historically, calendars emerged in ancient Mesopotamia, Egypt, and the Indus Valley, with Roman and Greek systems
In modern civil life, the Gregorian calendar is the de facto international standard, and ISO 8601 defines