One-Page Learn · The Halachos at a glance
קִדּוּשׁ הַחֹדֶשׁ
Sanctification of the New Month 8
Sefer Zemanim · Whole days instead of hours, ten months fixed permanently, and the two months left free to carry whatever is left over
793
Units past 29 and a half days in a lunar month
30
The day always established as Rosh Chodesh
2
Months whose length is not fixed in advance
354
Days in a year of six full and six lacking months
Part 1Days, never hours
- No month begins in the middle of a day. A lunar month is 29 and a half days plus 793 units, yet Rosh Chodesh may not begin mid day, since the Oral Tradition reads the verse to mean that you count the days of a month and do not count its hours. (8:1)
- Hence lacking and full months. A lacking month has only 29 days even though the lunar month runs several hours longer, and a full month has 30 even though the lunar month runs several hours shorter, so that months are reckoned in complete days. (8:2)
- The 793 units are what unbalance the year. Were the month exactly 29 and a half days there would be six full and six lacking months and exactly 354 days to a lunar year; it is the units above the half day, accumulating into hours and then days, that give some years more full months and others more lacking ones. (8:3)
RememberThe remainder is never discarded, only postponed.
Part 2Ten months fixed, two months free
- The thirtieth day is always Rosh Chodesh. If the month is lacking, the thirtieth day is Rosh Chodesh of the coming month; if it is full, the thirtieth is Rosh Chodesh yet counted as completing the month that ended, and the thirty first is Rosh Chodesh and the day the new month is reckoned from. (8:4)
- Tishrei full, Tevet lacking, then alternating. Tishrei is always full and Tevet always lacking; from Tevet the months alternate, Shevat full, Adar lacking, Nisan full, Iyar lacking, Sivan full, Tammuz lacking, Av full, Elul lacking, and in a leap year the first Adar is full and the second lacking. (8:5)
- Only Marcheshvan and Kislev are left open. Both may be full, both may be lacking, or Marcheshvan lacking and Kislev full; these give a year of complete months, a year of lacking months, and a year whose months proceed in order. (8:6)
RememberTen months were settled before the year began; the entire flexibility of the calendar sits in Marcheshvan and Kislev.
Part 3Reading the year from two weekdays
- Count between the two Rosh HaShanahs. In an ordinary year, count the days between this year's Rosh HaShanah and next year's, excluding both: two days means lacking months, three means the months proceed in order, four means complete months. (8:7)
- A leap year runs on different numbers. In a leap year four days between means lacking, five means in order, and six means complete, so the very same count of four reverses its meaning. (8:8)
- The Rambam's own worked example. Rosh HaShanah on Thursday in an ordinary year with the following Rosh HaShanah on Monday gives three days between, so the months proceed in order; if the following one falls on Tuesday, the months are complete. (8:9)
- Signs to check yourself against. Rosh HaShanah on Tuesday means the months always proceed in order, ordinary year or leap; on Shabbat or Monday they never do; on Thursday an ordinary year cannot be lacking and a leap year cannot proceed in order. (8:10)
RememberFour days between means the fullest year in an ordinary year and the emptiest in a leap year, so ask what kind of year it is before you read the count.