Halachah 1 – Why Months Are Counted Only in Whole Days
The lunar month lasts approximately 29 days, 12 hours, and 793 parts, as explained previously. Since the Torah requires months to be counted in complete days, a month cannot begin in the middle of a day. Therefore, the calendar must use only whole days, not fractions of days.
Halachah 2 – Two Possible Month Lengths
Because partial days are not counted, every month is either:
- 29 days (a deficient month, chaser)
- 30 days (a full month, malei)
Even though the true lunar month is slightly longer than 29½ days, months must be rounded into complete days.
Halachah 3 – Why Month Lengths Cannot Alternate Perfectly Forever
If the lunar month were exactly 29½ days, the calendar would simply alternate:
- 30 days
- 29 days
- 30 days
- 29 days
throughout the year.
However, because each lunar month contains an additional 793 parts beyond the half-day, those extra fractions accumulate over time. As a result, some years require more full months and others require more deficient months.
Halachah 4 – One-Day and Two-Day Rosh Chodesh
The thirtieth day always has some status of Rosh Chodesh:
- If the previous month was 29 days, day 30 becomes the only day of Rosh Chodesh.
- If the previous month was 30 days, day 30 serves as the final day of the old month and also as the first day of Rosh Chodesh, while day 31 becomes the principal Rosh Chodesh and the first day counted in the new month.
Therefore, some months have:
- One-day Rosh Chodesh
- Two-day Rosh Chodesh
Halachah 5 – The Fixed Pattern of Most Months
Most months follow a fixed pattern:
| Month | Length |
| Tishrei | Always Full (30) |
| Tevet | Always Deficient (29) |
| Shevat | Full (30) |
| Adar | Deficient (29) |
| Nisan | Full (30) |
| Iyar | Deficient (29) |
| Sivan | Full (30) |
| Tammuz | Deficient (29) |
| Av | Full (30) |
| Elul | Deficient (29) |
In a leap year:
- Adar I is always full.
- Adar II is always deficient.
Halachah 6 – The Special Role of Cheshvan and Kislev
Only two months vary:
- Cheshvan
- Kislev
These months determine the overall type of the year.
Three possibilities exist:
- Deficient Year (Chaserah)
- Cheshvan = 29
- Kislev = 29
- Regular Year (Kesidran)
- Cheshvan = 29
- Kislev = 30
- Complete Year (Sheleimah)
- Cheshvan = 30
- Kislev = 30
Halachah 7 – Determining the Type of a Regular Year
To determine whether a regular year is deficient, regular, or complete:
- Determine the weekday of Rosh Hashanah.
- Determine the weekday of the following Rosh Hashanah.
- Count the difference between them.
For a regular (12-month) year:
| Difference | Year Type |
| 2 weekdays | Deficient |
| 3 weekdays | Regular |
| 4 weekdays | Complete |
Halachah 8 – Determining the Type of a Leap Year
For a leap year:
| Difference | Year Type |
| 4 weekdays | Deficient |
| 5 weekdays | Regular |
| 6 weekdays | Complete |
The extra month shifts the calculation by two days.
Halachah 9 – Examples
The Rambam illustrates:
- If Rosh Hashanah falls on Thursday and the next year begins on Monday, the difference is three weekdays, making the year kesidran (regular).
- If the next Rosh Hashanah falls on Tuesday, the year is sheleimah (complete).
- Other combinations similarly reveal whether Cheshvan and Kislev were full or deficient.
Halachah 10 – Helpful Calendar Rules
Several shortcuts help identify the year’s type:
When Rosh Hashanah Falls on Tuesday
The year is always kesidran (regular), whether ordinary or leap.
When Rosh Hashanah Falls on Monday or Shabbat
The year can never be kesidran.
When Rosh Hashanah Falls on Thursday
- A regular year can never be deficient.
- A leap year can never be regular.
These rules serve as checks against calculation errors.