In Hebrew-calendar rules, “molad zaken” names the postponement of Rosh Hashanah when the molad occurs at or after noon.
A molad is the calculated time of the Moon’s mean conjunction, used to determine when Tishrei begins. The fixed Hebrew calendar does not simply declare Rosh Hashanah on the calculated day: it applies deḥiyyot, or postponement rules, to keep the calendar workable.
Molad zaken literally means “old molad,” traditionally understood as a late or aged lunar conjunction. If the calculated molad of Tishrei falls at noon or later, Rosh Hashanah moves to the following day. This rule helps prevent the calendar date from being set too early in relation to the visible new moon.
A common mix-up is Lo ADU, a separate rule that postpones Rosh Hashanah when the calculated day is Sunday, Wednesday, or Friday. Molad tohu is the name for the calendar’s hypothetical first molad, not the noon postponement rule. The molad calculation uses a mean lunar month of 29 days, 12 hours, and 793 parts.