This month shall be to you . . . (ibid. 12:2) “G‑d showed Moses the new moon, and said, ‘When you see it like this, sanctify.’”⁶ There is a tradition from the Baal Shem Tov that a person should fast on the eve of each Rosh Chodesh until the molad.⁷ Tzror HaChayim, p. 10b

footnotes: ⁶ Rosh Hashanah 20a. ⁷ The astronomical moment when the new moon appears. The Baal Shem Tov is expounding on the word “sanctify,” in the Sages’ statement. That is, on the eve of Rosh Chodesh, you should sanctify yourself by fasting, until the appearance of the new moon.