1,415. Reciting Hallel

Hilchos Megillah v’Chanukah 3:6

Hallel is not only a rabbinic mitzvah on Chanukah. Rather, any time the whole Hallel is recited, it is a rabbinic mitzvah. There are 18 days each year on which it is a mitzvah to recite the Hallel in its entirety: eight days of Succos, eight days of Chanukah, the first day of Passover, and Shavuos. Hallel is not recited on Rosh Hashana or Yom Kippur because these are days of repentance, awe, and reverence, so extra celebration is deemed inappropriate. Hallel was not instituted on Purim, the reading of the Megillah fulfilling the same purpose.

Hilchos Megillah v’Chanukah 3:7

In a place where yom tov is celebrated for two days, Hallel is recited on 21 days: nine days of Succos, eight days of Chanukah, the first two days of Passocer, and the two days of Shavuos. Reciting Hallel on Rosh Chodesh is a communal custom, not a (rabbinic) mitzvah. Because of its status as a custom, some passages are omitted when Hallel is recited on Rosh Chodesh. A bracha is not recited because a bracha is not recited over a custom. One who prays alone does not recite Hallel on Rosh Chodesh but if he started it, he should finish it, omitting the passages that a congregation would omit. Similarly, on the middle and last days of Passover, Hallel is recited with the same omissions. [The rules in this halacha about not reciting a bracha over half-Hallel and not reciting it when davening alone may not reflect our familiar practices.]