Hallel on Rosh Chodesh

 https://ph.yhb.org.il/en/05-01-12/

There is a widespread custom to recite Hallel on Rosh Chodesh. Indeed, according to the letter of the law, there is no obligation to do so, because Hallel is required only on days that are called mo’ed (appointed times), and when work is prohibited. Rosh Chodesh is called a mo’ed, but work is permitted on the day. Still, the Jewish people have accepted the practice of saying Hallel on Rosh Chodesh, in order to give expression to the sanctity of the day, which is capable of elevating one to the level of singing God’s praises. To make it clear that the Hallel of Rosh Chodesh is based on custom, and not obligatory, we omit two paragraphs that are said when reciting the complete Hallel (the complete Hallel consists of chapters 113-118 of Tehillim, and we skip 115:1-11 and 116:1-11).