Hallel on Rosh Chodesh
Question
Why do we skip two paragraphs in hallel on Rosh chodesh ?
Answer
Shalom!
Thank you for your question!
The reason an abridged version of Hallel is recited on Rosh Chodesh is because Rosh Chodesh is not a full Yom Tov, on which a full Hallel is required, nor did a miracle happen on that day, which is another reason that full Hallel is required. As such, the recitation of Hallel on Rosh Chodesh has the status of a custom and not an outright obligation. Reciting Hallel on Rosh Chodesh was not part of the original Hallel enactment of our sages. It developed in a later period.
While common custom today is to abridge the Hallel by omitting that passages of “Lo Lanu” and “Ahavti” there were actually different customs in earlier generations on which passages to omit. The reason these two passage were chosen is because they are a reference to the Messianic era and the resurrection of the dead. By omitting these themes one clearly demonstrates that Hallel on Rosh Chodesh is a custom and not an obligation, as when Hallel is an outright obligation one would not fulfill the mitzva if these passages, and their themes, are omitted.
A related question is how it is permitted to recite a blessing when reciting Hallel on Rosh Chodesh if, as mentioned, Hallel on Rosh Chodesh is a custom and not a true obligation. Well, indeed, Sefardim do not recite a blessing when reciting Hallel on Rosh Chodesh for this exact reason, though in some Sefardic congregations the Chazzan recites the blessing on behalf of everyone. Ashkenazim, on the other hand, follow the ruling of Tosfot that one is permitted to recite a blessing when performing certain customs.
Source
Arachin 10b; see Rashi, Taanis 28b s.v. di’midalgi; Rambam, Hilchos Chanukah 3:7; OC 422:2.