- Calendar -

The Ancient Egyptians used three different kinds of calendar systems: the 'agricultural' calendar for day-to-day use, an astronomical calendar and a moon-calendar for certain rituals and events. 

With the 'agricultural' calendar, the year was divided in 3 seasons of 4 months: akhet (inundation), peret ("the coming forth of the land out of the inundation") and shemu ("harvesting"). Each month counted 30 days, which means that the year had a total number of 360 days. To these were added 5 "epagomenal" days, which were considered as the birthdays of Osiris, Horus, Seth, Isis and Nephthys.

The months were named as follows (the pronunciation of these names has been based on Coptic) : 

• Akhet I = Thot, named after the god Thot
• Akhet II = Paopi 
• Akhet III = Athor, named after the goddess Hathor
• Akhet IV = Khoiak 
• Peret I = Tobi 
• Peret II = Mekhir 
• Peret III = Pnamenoth 
• Peret IV = Pharmuthi 
• Shemu I = Pakhons, named after the god Khonsu
• Shemu II = Paoni 
• Shemu III = Epep 
• Shemu IV = Mesore, "the birth of Re"
   

Many temples would keep a calendar with a list of rituals and festivals to be held at specific dates. In the temple of Esna, for instance, this list was inscribed on the many columns of the pronaos of the temple. In the temple of Horus at Edfu, the month of Khoiak was especially rich in festivals. 

With 365 days per year, this calendar would slowly veer away from the true year: every 4 years, the "agricultural" calendar would advance 1 day. After some time, the "inundation"-season would no longer coincide with the real inundation. Every 1460 years, the "agricultural" calendar would coincide with the true year for 4 years. 

To deal with this discrepancy, the Ancient Egyptians used an astronomical way of measuring time: they observed the rising of the star Sirius along with the sun, which always coincided with the beginning of the inundation. Knowing at what "agricultural" date this event occurred, has allowed us to have a fairly exact means of calculating dates according to our own calendar system. 

Certain rituals, especially those involving Osiris and the lunar deity Khonsu, had to be performed at given times and during specific lunar phases. To calculate when such rituals needed to be performed, the Ancient Egyptian priests kept a calendar of lunar phases, according to which a month coincided with one lunar cycle. Each day was either named after a lunar phase or after a ritual.