From the beginning of organized civilization, humans have always had a need to organize and document time as well as plan for future dates. Calendars, by their very nature, allowed people to plan and order events as well as a way to communicate about the events relative to one another. Calendars originally primarily revolved around agriculture or religious occurrences, and many ancient calendars were based on the phases of the moon or other natural phenomenon.

Mesoamerican Calendars
The ancient civilizations of Mesoamerica primarily developed calendars and calendrical systems to organize and schedule religious rituals and social events. While each culture had its own system, many used a ritual calendar with seemingly no correlation to any type of lunar, solar or agricultural cycle. The most well known of the ancient Mesoamerican calendars are the Mayan and the Aztec calendars.

• The Mayan Calendar
• Ancient Mayan Calendars
• The Aztec Calendar
• Introduction to the Aztec Calendar
• Aztec and Mayan Calendar

Jewish Calendar
The Jewish calendar is considered an ancient calendar, but it is also still used for religious planning within the Jewish community and, in fact, serves as the national calendar of Israel. This calendar is based on solar and lunar patterns as well as many other complicated factors and cannot be predicted in advance from year to year. It typically uses 12 months, although a leap month is added approximately every third year depending on weather patterns.

• The Jewish Calendar
• Background and History of the Jewish Calendar
• Hebcal Jewish Calendar
• Judaism 101: Jewish Calendar
• Jewish Holiday Listing

Islamic Calendar
The Hijiri calendar used by Muslims for religious purposes is a lunar calendar, with each month beginning when the moon is first spotted in Mecca after a new moon. This means that any Islamic calendar that is printed in advance is merely an estimate and cannot be considered 100 percent accurate. This tradition is based on the Islamic sacred text, the Quran, which requires proper observance of religious rituals according to this calendar. Because it is based on lunar patterns, the Islamic year is made of 12 months but is somewhat shorter than a traditional calendar.

• The Islamic Calendar
• Islamic Finder: Calendar Converter
• Saudi Arabia’s Islamic Calendar
• Islamic Calendar for Makkah

Chinese Calendar
The Chinese calendar is one of the oldest calendars still in use, with tradition stating that it was invented in 2637 B.C. There is record of it dating back to at least the 14th century B.C. It is still used in China and in Chinese communities worldwide to determine the scheduling of social and cultural holidays and festivals. The Chinese calendar is solar and lunar in nature, and is based on observations of the longitude of the sun as well as the phases of the moon. It typically has 12 months, with a leap month added every several years.

• The Chinese Calendar
• Early Chinese Calendars
• WesternChinese Calendar Converter
• Mathematics of the Chinese Calendar

Indian Calendar
In 1957, a group known as the Calendar Reform Committee was formed within the Indian government to establish a National Calendar of India. The resulting calendar aligned the Indian leap years with the leap years of the Gregorian calendar although it does retain traditional Indian month names. The start of each month is based on lunar phenomenon and does not align with the beginning of the Gregorian months.

• The Indian Calendar
• Panchangam: The Indian Calendar
• Indian Calendars: Mathematics
• The Hindu Religious Calendar

Zoroastrian Calendar
Zoroastrians use 3 different calendars, with a certain level of controversy about which is accurate. These are the Fasli, the Shahanshahi and the Qadimi calendars. In all 3 calendars, each day is observed in honor of a specific spiritual being, and certain prayers are offered for that being. Sacred days occur when the month and the day are both presided over by the same being. The most popular Zoroastrian religious calendar is the Shahanshahi, also called the Shenshai.

• The Zoroastrian Religious Calendar
• The 3 Zarathushtrian Calendars
• Months and Days of the Zoroastrian Calendar
• The Shenshai Calendar

Native American Calendars
While many tribal groups in North America had their own versions of a calendar, they had a few things in common. These calendars were primarily agricultural in nature, and the year typically began in the spring. New growth and the budding of plants was considered a new beginning my many Native American cultures, and this was honored through their calendar systems.

• Native Net: Native American Calendars 
• Native American Calendars
• Calendars of Native Americans: Timekeeping Methods of Ancient North America
• Native American Calendars  How the First People in the Americas Kept Up with Time

Additional Resources

• Calendars through the Ages has information on dozens of ancient and current calendars.
• Ancient Calendars discusses calendars from ancient civilizations who no longer exist or whose calendars are no longer in use.
• A Walk Through Time: Early Clocks is hosted by the National Institute of Standards and Technology.
• A Walk Through Time: Ancient Calendars is a look at the history of modern calendars and how dates have been tracked through history, provided by the National Institute of Standards and Technology.
• A Brief History of Clocks and Calendars describes the necessity of tracking time and date, and how this guides all human cultures.
• A Religious Interfaith Calendar is presented by the BBC.