ChristianAnswers.Net WebBible Encyclopedia
It is called “The Egyptian Hallel,” because it was chanted in the temple whilst the Passover lambs were being slain. It was chanted also on other festival occasions, as at Pentecost, the feast of Tabernacles, and the feast of Dedication. The Levites, standing before the altar, chanted it verse by verse, the people responding by repeating the verses or by intoned hallelujahs. It was also chanted in private families at the feast of Passover.
There is also another group called “The Great Hallel,” comprehending Psalms 118-136, which was recited on the first evening at the Passover supper and on occasions of great joy.