Isese Day is a regional public holiday observed in the Nigerian state of Osun on August 20th each year.
The day is set aside by the Osun government for traditional worshipers to celebrate their deities. The day is observed as the culmination of a week of traditional religious activities and festivities.
 

Isese (the Yorùbá word for Tradition) Day celebrates the Yorùbá traditions and religion in a display of culture and spirituality with parties, festivals, parades, lectures and display of other traditional apparels to attract tourists from other parts of the world.

The Yorùbá make up 21% of Nigeria's population.
As well as in Osun state, Isese day is celebrated by Yorùbá diaspora communities around the world.
 

A former governor of  Osun, Mr Rauf Aregbesola,  in 2013, approved August 20th as a public holiday in the state for Traditional Worshipers, under Section 2 (2) of the Public Holidays Act CAP P40 Laws of the Federal Republic of Nigeria 2010.

Isese Day was first declared as a public holiday by the Osun State government in 2014. Banks, government offices and other establishments in Osun State will be closed on this day.
Although the actual day is a state holiday, week-long festivities leading up to August 20th are celebrated in Osogbo, the capital of Osun state.
The appropriate greeting for Isese Day is "Isese l'agba" and the response is "Isese l'agba gbogbo wa".