It is customary for women to refrain from performing labor on Rosh Hodesh because they did not sin at the Golden Calf. The strong faith of women relates to their spiritual source, the Sefira of Malkhut, which is symbolized by the moon. The moon receives its light from the sun, and its light depends on the sun alone. Women perceive their relationship to the Creator in the same way.

Rosh Hodesh was given to women as a reward for having resisted participating in the sin of the Golden Calf:

It is necessary to understand: Why was the festival of Rosh Hodesh given specifically to women as a day on which they are prohibited from performing labor? Are not women exempt from positive, time-bound mitzvot [and therefore it is odd that they would have such a specifically time-bound custom]? There are many reasons for this…In Pirkei deRabbi Eliezer there is a reason for this: When the mixed multitude [gentiles who joined the children of Israel for the exodus from Egypt] caused Israel to go astray and make the Golden Calf, and said to them: “Remove the gold rings” (Exodus 32:2), the women did not contribute, and therefore they were given a reward, the holiday of Rosh Hodesh.

The spiritual explanation for the women’s refusal to participate in the sin of the Golden Calf relates to their spiritual source:

The women did not accept that thinking [behind the sin of the Golden Calf], because women are of the feminine realm, whose source is from [the Sefira of] Malkhut…and Malkhut is the source of all things that have been created, and a revelation from it can be felt in a person’s soul. The women felt this [revelation of divine providence], and did not accept that thinking, and did not remove their gold rings.

Rosh Hodesh symbolizes the connection between the moon and the sun, and at the same time, between the world and the divine light:

Because on Rosh Hodesh the moon receives reflected light from the sun, and its light essentially comes from the sun. The moon represents Malkhut, which is God’s name Elohim [symbolizing God’s revelation in the world], which receives all its power from the Tetragrammaton [symbolizing God’s mastery of the world], and this relates to the unity of the Tetragrammaton and Elohim. Therefore women, whose source is from Malkhut, the name Elohim, and who did not put their faith in the Golden Calf, were rewarded with the cessation of labor on Rosh Hodesh, which is about the unity of the Tetragrammaton and Elohim. (Rabbi Shmuel Schneersohn, Torat Shmuel 5640, vol. 1, Noah)

Rejoice with the Blessing of the Moon

Rejoice, rejoice, with the blessing of the moon

The renewal of the moon, when my heart is joyous and we will pummel our foes by the light of the Shekhina.

He who acts in truth, His actions are truth, and it will be completed in the completion of its construction.

The lady shall be renewed with the raising of the crown, all the way to the crown [the Sefira of Keter] of glory [the Sefira of Tiferet] with the glory [Tiferet] of understanding [Bina].

O He who brings salvation, dissolve the oath, come and reveal Yourself in the heavenly city.

The renewal of souls will be in all the worlds, the Names will shine all around and at [every] corner.

Implore now before God: May He send the redeemer, gathering Israel whom He has acquired.

Lord, hear now this plea and send now reprieve, please save, please bring success.

Further reading: Concerning the blessing of the New Moon and how it is performed, see A Concise Guide to Halakha, p. 240.