In the traditional Igbo calendar a week (Igbo: Izu) has 4 days (Igbo: Ubochi) (Eke, Orie, Afọ, Nkwọ), seven weeks make one month (Igbo: Ọnwa), a month has 28 days and there are 13 months a year. In the last month, an extra day is added. The traditional time keepers in Igboland are the priests or Dibia.

No. Months (Ọnwa) Gregorian equivalent
1 Ọnwa Mbụ (February–March)
2 Ọnwa Abụo (March–April)
3 Ọnwa Ife Eke (April–May)
4 Ọnwa Anọ (May–June)
5 Ọnwa Agwụ (June–July)
6 Ọnwa Ifejiọkụ (July–August)
7 Ọnwa Alọm Chi (August to early September)
8 Ọnwa Ilo Mmụọ (Late September)
9 Ọnwa Ana (October)
10 Ọnwa Okike (Early November)
11 Ọnwa Ajana (Late November)
12 Ọnwa Ede Ajana (Late November to December)
13 Ọnwa Ụzọ Alụsị (January to early February)

The names of the days have their roots in the mythology of the Kingdom of Nri. Eri, the sky-descended founder of the Nri kingdom, had gone on to break the mystery of time and on his journey he had saluted and counted the four days by the names of the spirits that governed them, hence the names of the spirits eke, orie, afọ and nkwọ became those of the days of the week. The days also correspond to the four cardinal points, Afọ corresponds to north, Nkwọ to south, Eke to east, and Orie to west. These spirits, who were fishmongers, were sent down by Chukwu (Great God) in order to establish markets throughout Igboland which they did by selling fish.

While there are four days, they come in alternate cycles of "major" and "minor", giving a longer eight day cycle

An example of a month: Ọnwa Mbụ

Eke Orie Afọ Nkwọ
1 2
3 4 5 6
7 8 9 10
11 12 13 14
15 16 17 18
19 20 21 22
23 24 25 26
27 28

The Igbo calendar is not universal, and is described as "not something written down and followed ... rather it is observed in the mind of the people.

Source: https://en.wikipedia.org/wiki/Igbo_calendar