The 'Japji' is the first message that sprung out like nectar from Guru Nanak's lips as he emerged from the river after 3 days. Guru Nanak remained immersed in the River for 3 days. It is said that it takes 3 days for the ego to totally disappear. It is then that one experiences the Ultimate. That is the reason why maybe the 'Marka' and/or 'baithak' and/or 'Chautha' is done on or after 3 days after a person's death.

http://www.dalsabzi.com/message/gurunanak1.htm