The traditional Yoruba week is a 4 days' week. But it is often mentioned as a 5 steps cycle as the 5th step is the same as the first one. As a cycle, it has no beginning and no end.
In Ọ̀wónrín Ọjọ́ Ọ̀sẹ̀ (Ọ̀wọ́nrín Ìká), Ọ̀rúnmìlà is said to be the owner of all the days of the week as he is the responsable of the worship time of each Òrìṣà. For this reason and also because market day is the same day as Ifá's day, so it's easy to remember, we often use Ọ̀sẹ̀ Ifá as the reference point of this cycle, especially the Babaláwo, Ìyánífá, and Ọmọ Awo.
Many Òrìṣà are worshipped on the day of Ifá . For example, Ọ̀ṣun is worshipped on this day because she is the wife of Ọ̀rúnmìlà.
Ajé can also be worshipped on this day appart from Monday. She is one of the many wifes of Ọ̀rúnmìlà. It is also the day of the Market wich is a good point for the Òrìṣà of wealth. Ọ̀rúnmìlà has a lot of friends and relatives so this is generally a very animate day.
In terms of seniority, the first one to be in charge of a specific day is Ọbàtálá , as the elder Òrìṣà. On another side, Ṣàngó, as the last revealed Òrìṣà obtained his own at last after fighting for it with his sacred stones (Jàkúta means "Fight with stones" and is also a surname for Sàngó, that is why we usually say Ọ̀ṣẹ̀ Jàkúta instead of simply saying Ọ̀ṣẹ̀ Sàngó).
The 4 days of the week are:
The other Òrìṣà(s) will be celebrated on any of those days depending of their affinity with the owners of the days and the customs of the worshipper's family.
Ajé ( Òrìṣà of wealth) is often celebrated on the Ifá 's day but is also celebrated every Monday ( Ọjọ́ Ajé ) on a 7 days periodicity.