Haflagah (Interval)
A woman anticipates beginning her menses when an interval (haflagah) has elapsed which is the same as that which separated her previous two periods. This is called veset hahaflagah. It is one of the onot perishah (periods of separation) during which marital relations are forbidden and on which a bedikah (internal examination) must be performed to check whether menstruation has begun.
The following example illustrates how a veset hahaflagah is calculated:
A woman begins to menstruate on the 15th of Nisan, and her next menses begin 26 days later, on the 10th of Iyar. Her haflagah thus falls after another 26-day interval, on the 6th of Sivan. Note that the first day of menses is counted both as the last day of the previous interval and as the first day of the next interval. (This is the common practice. Chabad, however, counts the interval between the last day of one menstrual period and the first day of the next).
The onat perishah, either daytime (sunrise to sunset) or night-time (sunset to sunrise), corresponds to the onah of the immediately preceding menses. Thus, if a woman began to menstruate during the day on the 10th of Iyar, she will observe the veset hahaflagah during the day on the 6th of Sivan.
Nissan
| Rishon | Sheini | Shlishi | Revi'i | Chamishi | Shishi | Shabbat |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 8 | 9 | 10 | 11 | 12 | 13 | 14 |
15 (1) |
16 (2) |
17 (3) |
18 (4) |
19 (5) |
20 (6) |
21 (7) |
| 22 (8) | 23 (9) | 24 (10) | 25 (11) | 26 (12) | 27 (13) | 28 (14) |
| 29 (15) | 30 (16) |
Iyar
| Rishon | Sheini | Shlishi | Revi'i | Chamishi | Shishi | Shabbat |
| 1 (17) | 2 (18) | 3 (19) | 4 (20) | 5 (21) | ||
| 6 (22) | 7 (23) | 8 (24) | 9 (25) | 10 (26=1) |
11 (2) |
12 (3) |
| 13 (4) |
14 (5) |
15 (6) |
16 (7) | 17 (8) | 18 (9) | 19 (10) |
| 20 (11) | 21 (12) | 22 (13) | 23 (14) | 24 (15) | 25 (16) | 26 (17) |
| 27 (18) | 28 (19) | 29 (20) |
Sivan
| Rishon | Sheini | Shlishi | Revi'i | Chamishi | Shishi | Shabbat |
| 1 (21) | 2 (22) | 3 (23) | 4 (24) | |||
| 5 (25) | 6 (26) |
7 | 8 | 9 | 10 | 11 |
| 12 | 13 | 14 | 15 | 16 | 17 | 18 |
| 19 | 20 | 21 | 22 | 23 | 24 | 25 |
| 26 | 27 | 28 | 29 | 30 |
If a woman does not menstruate at the end of the interval as anticipated, she no longer anticipates that interval. Thus, if she does not begin to menstruate after 26 days on the 6th of Sivan, but only two days later after a 28-day interval on the 8th of Sivan, she observes her next veset hahaflagah 28 days later on the 5th of Tammuz. Similarly, if she began to menstruate two days earlier than expected, on the 4th of Sivan after only a 24-day haflagah, her next veset hahaflagah will be 24 days later, on the 27th of Sivan.
However, a woman who has established a veset kavua (established cycle) to begin menstruation after a specific interval (in the case of haflagot, this requires four consecutive menstrual periods separated by three equal intervals) would continue to anticipate her menses after her established interval, counting from the day she began menstruating, even if for a month or two her menses did not begin at the expected time.
We have described the most common method of calculating the haflagah, but other methods exist. One such custom is to count the exact number of onot. Each community should follow its own custom.
