Haflaga is the interval of onot between a successful hefsek tahara and the subsequent menstrual flow. The haflaga is customarily counted beginning with the night onah immediately following the hefsek tahara and terminating with the onah on which the following flow begins. Each date consists of two onot – a night onah followed by a day onah. You will have a count of two onot for each date, beginning with the date immediately following your successful Hefsek Tahara and ending your count with the onah upon which your flow begins. Calculating by this method, the haflaga number is an odd number if the flow begins at night, and the separation will be during a night onah. The haflaga number is an even number if the flow begins during the day, and the separation will be during a day onah. The haflaga numbercan be calculated only once there is a successful Hefsek Tahara and then the onset of a subsequent period. The recording of the haflaga onah/ot onto the calendar, however, can only occur once the subsequent Hefsek Tahara has been successfully complete
Some people compute the haflaga by counting the onot from the onah immediately following the hefsek taharah up to and including the onah immediately BEFORE the new flow. When figuring the haflaga onah/ot, they abstain on the onah that immediately follows the end of their count. This coincides with the haflagh onah designated by the method explained in the main text. However, calculating by this alternative method will give an even number indicating separation during a night onah and an odd number indicates separation on a day onah. Regardless of how one calculates the haflaga, the onah of separation remains the same, only the number is different
Mark your calendar with “HT” on the day you perform and complete a successful hefsek tahara before sunset, as explained above.
Count out the onot, each day consisting of two (night and day), beginning with first date following the successful hefsek tahara of your previous period. End the count on the date your next period begins. Mark this number down next to the “P” noting the onset of your period. This is your haflaga number. Remember that you will have an odd number for a period that began on a night onah and an even number for the period that began during the day onah.
When you complete your next hefsek tahara, mark the new “HT” onto your calendar on the date it was done (as explained above).
Beginning with the date immediately following that new hefsek tahara, count the same number of onot (each day consisting of two).
Mark the date upon which your count ends with the haflaga number. Make note of the time your period began that created this haflaga number, including a.m. or p.m. This is your haflaga onah of separation.
Repeat this process with each haflaga number on your calendar. (see below What to Carry Over)
Each time a haflaga number is determined, it is always recorded for the current month. It remains on your calendar and is carried over monthly until a subsequent period gives you a greater haflaga number. At that point the greater number cancels all previous haflaga numbers that are smaller than it.
In other words, if your flow comes after any date/s marked with haflaga numbers (haflaga onot), the haflaga numbers corresponding to those dates are cancelled, marked with an “X” and are not carried over to the following month.
What if your flow comes after some date/s marked with haflaga numbers (as explained above), but before other, later, date/s on your calendar marked with haflaga numbers?
Those later date/s marked with haflaga numbers are disregarded for the present and do not require separation and bedikot. However, the haflaga numbers on those later date/s are not cancelled. Those haflaga numbers are carried over to the following month and must be marked onto new dates recalculated from the newest hefsek tahara. If your flow begins on a date marked with a haflaga number, its corresponding number is also carried over to the following month. The next date of this haflaga number will be a different one. You will calculate the new date after the next hefsek tahara has been successfully completed.
To paraphrase the above, all previous haflaga numbers greater than or equal to the current month's haflaga number are carried over to the following month. The dates corresponding to those haflaga numbers will be recalculated from the newest hefsek tahara. Those dates will be marked with the corresponding haflaga numbers as explained. Any numbers lower than the current month's haflaga number are cancelled.
You must abstain from marital relations during the full designated onah of every Hebrew date marked with a haflaga number, as long as a new flow did not precede it. Remember that any date marked with an odd number (if counting by the method in the main text – opposite if counting by the method in the Note) means a separation from sunset to sunrise of that Hebrew date (i.e. if the date marked with an odd number is a Monday, then separation would be from sunset Sunday to sunrise Monday morning). Any date marked with an even number means a separation from sunrise to sunset of that Hebrew date (i.e. if the date marked with an even number is a Monday, then separation would be from sunrise Monday morning to sunset Monday evening). It is praiseworthy to be strict and refrain even from embracing and kissing, and even more so from sharing the same bed. The Alter Rebbe states in his Shulchan Aruch, "He who is stringent in this matter will be blessed." (A.R.S.A. chapter 184, note 6
Make a bedikah on each onah on which you anticipate a flow. Your bedikah should be made preferably just AFTER the time of day your flow began that created the Veset Haflagah or sometime later on, but before the end of the onah. If the bedikah is clear or white, physical intimacy may resume only after the onah has ended. (p. 37 Kitzur Dinei Tahara)