"The first of Nissan is the New Year for festivals" (Rosh Hashanah 2a). There is no holiday that begins on the first of Nissan. Rather, the Gemara explains, the first of Nissan is the month in which the first of the holidays (Pesach) occurs. While we observe the technical details of Pesach for seven (or eight) days, the holiday of Pesach fills an entire month. This celebration is even explicit in the Torah itself: "The month that you left Egypt" (Shemot 23:15); "Guard the month of the spring and make Pesach" (Devarim 16:1).