Hallel

Hallel means “Praise”
It is a group of Psalms 113–118 that is recited or sung on holidays and Rosh Chodesh. The complete Hallel is recited on the three major holidays, Passover first night, Sukkot, and Shavuot. These Psalms praise Hashem and connect us to the Shechina.
On the night of the Seder, there is a great revelation of Light and we recite the entire Hallel. This Light is concealed the next day because Israel did not achieve complete purity, and for that reason, during the following days of Passover, we recite the Hallel with a few skips as we do on Rosh Chodesh which is not a major holiday. On Sukkot, we recite the complete Hallel, every day of the holiday.