One-Page Learn · The Halachos at a glance
קִדּוּשׁ הַחֹדֶשׁ
Sanctification of the New Month 7
Sefer Zemanim · The postponements: the three weekdays Rosh Chodesh Tishrei never takes, the hour that moves it, the two numbered thresholds, and the reason the Rambam actually gives
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Weekdays Rosh Chodesh Tishrei never takes
4
Instances that move the day off the molad
204
Parts that send an ordinary year to Thursday
589
Parts that send a post leap year to Tuesday
Part 1Three days that are always refused
- Lo ADU rosh. Rosh Chodesh Tishrei is never established on a Sunday, a Wednesday or a Friday, whatever the calculation yields; the symbol for the three is aleph, dalet, vav. (7:1)
- Move it one day forward. A molad on Sunday gives Rosh Chodesh on Monday, a molad on Wednesday gives Thursday, and a molad on Friday gives Shabbat. (7:1)
- The permitted days alternate with the refused ones. Tuesday we fix, Wednesday we defer, Thursday we fix, Friday we defer, Shabbat we fix, Sunday we defer, Monday we fix. (7:7)
RememberThree of the seven doors are shut before the arithmetic is even consulted.
Part 2The hour, and two exact thresholds
- Molad zaken. If the conjunction falls at midday or after midday, Rosh Chodesh is established on the following day; the example given is a molad on Monday six hours after daybreak or later, which fixes the month on Tuesday. (7:2)
- Before midday, even by one part, the day stands. A conjunction earlier than midday fixes Rosh Chodesh on the day of the molad itself, provided that day is not one of the three refused weekdays. (7:2)
- GaTaRaD, three, nine, two hundred and four. In an ordinary year, a molad on the night of the third day at nine hours and 204 parts of the tenth hour or later moves Rosh Chodesh from Tuesday to Thursday. (7:4)
- BeTU'TaKPaT, two, fifteen, five hundred and eighty nine. In a year following a leap year, a molad on Monday at three hours and 589 parts of the fourth hour after daybreak or later moves Rosh Chodesh from Monday to Tuesday. (7:5)
RememberA part is a 1080th of an hour, and the whole year's festivals turn on it.
Part 3Why the court overrules its own reckoning
- Four instances, then the day of the molad stands. The day of the conjunction is always the day of the fixing except in four cases: a molad on one of the three refused weekdays, a molad at midday or later, GaTaRaD in an ordinary year, and BeTU'TaKPaT in a year following a leap year. (7:6)
- The reckoning is a mean, not a true position. The calculation tracks the gathering of the sun and the moon by their mean motion rather than their true place in the sphere, so a day of fixing and a day of deferring were instituted in order to meet the true conjunction. (7:7)
- The same reason stands behind the other postponements. The Rambam brings proof: a molad on the night of the third day is deferred to Thursday, and often the moon is not seen on Thursday night nor even on Friday night, showing that the true conjunction was only on Thursday. (7:8)
RememberThe Rambam's stated reason for the postponements is astronomical, not the familiar practical ones.