Order in the Laws of Menstrual Cycle Days

Can you provide me with the order of separation time and inspection time on the third day, the day of the menstrual cycle, and the interval day?

Hello,

The day of the menstrual cycle is a general term for days when there is a concern of seeing blood, and therefore one must abstain from their husband and also perform an inspection.

On each day of the menstrual cycle, it is customary to perform two inspections: one at the beginning of the period of abstention and one at the end. However, if it is difficult, one inspection is sufficient.

The main days of the menstrual cycle are three: A. The interval day. This means calculating the days between the last day of the menstrual cycle and the previous one. For example, a woman who saw blood on Sunday of the month of Nisan, and then on Sunday of the month of Iyar, the number of interval days is 31 (including both days of the cycles). The next day of the menstrual cycle will be on Monday of Iyar, which will be the 31st day from the last cycle.

B. The day of the month. One must also abstain on the same date of the last sighting in the following month (calculated only according to the Hebrew calendar). For example, in the case mentioned in the previous section, the last sighting was on Sunday of the month of Iyar, so one must also abstain on Sunday of the month of Sivan.

C. The average period. The 30th day from the last sighting, as it is natural for women to see on the 30th day from the last sighting. A woman who has not established another order should be concerned that she will see on the 30th day as is commonly the case. The 30th day is calculated including the day of sighting and the day of abstention. For example, a woman who saw on Sunday of Iyar should abstain and inspect on Sunday of Sivan, which is the 30th day from her last sighting.

As you have noticed, often the day of the month and the 30th day are the same period.

Shulchan Aruch, Yoreh De'ah, Siman 189