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The Igbo calendar (
Such a calendar was presented by Onasanya (2009) in his The Urgency of Now!: Building a True Nigerian Nation.
 Many parts of this calendar are named for or dedicated to certain spirits (
Although worship and spirit honoring was a very big part in the creation and development of the Igbo calendar system, commerce also played a major role in creating the Igbo calendar. This was emphasized in Igbo mythology itself. An example of this is the Igbo market days of which each community has a day assigned to open its markets, this way the Igbo calendar is still in use.
Some Igbo communities have tried to adjust the thirteen month calendar to twelve months, in line with the Gregorian calendar. 
The calendar is neither universal nor synchronized, so various groups will be at different stages of the week, or even year. Nonetheless the four-eight day cycle serves to synchronize the inter-village market days, and substantial parts (for example the Nri kingdom) do share the same year-start.
Igbos generally have four market days, namely: eke, orie, afor and nkwo. The market days according to the
In various parts of Igboland, each community has a market named after the aforementioned four market days, e.g., Eke market, Afor market.