The first of Nisan’s festivals was the Passover, originally celebrated in Egypt; it came on the 14th of the month and included the sacrifice of the paschal lamb. (Ex 12:2-14; Le 23:5; De 16:1) The following day was the beginning of the week-long Festival of Unfermented Cakes, running from the 15th to the 21st of the month.
https://www.jw.org/en/library/books/Insight-on-the-Scriptures/Nisan/