The Hebrew yerach refers to the lunar month, the basic unit of the Hebrew calendar. The Hebrew calendar is fundamentally lunar, with each month beginning at the new moon. Yerach appears alongside chodesh (H2320, the new moon/month) as a near-synonym, though yerach more specifically emphasizes the full lunar cycle of approximately 29-30 days.
Israel's sacred calendar was divinely structured around the moon's cycles (Genesis 1:14 β lights for signs, seasons, days, years). New Moon celebrations were significant worship occasions (Numbers 10:10; 28:11-15). Psalm 104:19 declares God made the moon for appointed seasons. The Passover, Pentecost, and Feast of Tabernacles were all governed by lunar dating. In this way yerach connects astronomical order to the covenant community's rhythm of worship, rest, and feasting.