Daf Yomi - Taanis 28 - Hallel on Chanukah is a Torah Requirement

The Gemora inquires as to why the Mishna did not mention that there was no ma’amad on the first of Nissan for they recited Hallel and there was a Mussaf sacrifice and a wood offering. Rava answers that this Mishna is an indicator that the Hallel which is sung on Rosh Chodesh is not based upon a biblical ordinance and therefore the ma’amad is not suspended. This is based upon the words of Rabbi Yochanan in the name of Rabbi Shimon ben Yehotzadak who states: "Eighteen times during the year an individual may recite the whole Hallel, and they are: On the eight days of Sukkos, on the eight days of Chanukah, on the first day of Pesach and on the first day of Shavuos. While in exile, however, one may recite it twenty-one times during the year, namely: On the nine days of Sukkos, on the eight days of Chanukah, on the first two days of Pesach and on the two days of Shavuos." Rosh Chodesh is not mentioned here, indicating that it is only a custom and therefore it will not suspend the ma’amad.

Rashi states that the obligation to recite Hallel on Chanukah is similar to a Biblical obligation since it was established in accordance with the practice instituted by the prophets of reciting Hallel whenever the Jewish people are saved from a dangerous situation.

Although we generally relate to the holiday Chanukah as being of Rabbinic origin, Chasam Sofer explains that the obligation to establish a festival on the day on which a miracle occurred is, in fact, a Torah requirement. Chasam Sofer writes, “therefore observing the days of Purim and Chanukah are indeed a Torah obligation and one who fails to do so is nullifying a positive precept in the Torah.” It is a long-standing Jewish custom to hold celebratory meals during Chanukah to provide an opportunity to sing and recite praises to G-d for the miracles He performed on our behalf. Doing so, explains the Chasam Sofer, is the fulfillment of a positive Torah commandment.

This last paragraph can be found here.