How and When
Here is a brief outline of the process of counting the days of nida and the process of mikvah renewal for a bride and subsequently for married women after each menstrual cycle. After a woman has menstruated, she checks with a white cloth, packs of which which can be obtained from the mikvah attendant, to be sure there is no more blood. This is colloquially called “checking,” and when she finds no blood it is often referred to in English as a “clean check.” Obviously, different women have different length menstrual periods, so how long it will take until she has a “clean check” will depend on her cycle. In Jewish law if a married woman has a period that lasts less than five days, she waits five minimum days before counting her “clean” days.
After cessation of menstrual bleeding a woman counts seven “white” days, during which she checks each day to be sure there is no blood and usually wears white undergarments. During this week one should call the mikvah to schedule an immersion on the night following the seventh white day. At the end of these seven days she can immerse in the mikvah. The night after a wife has gone to the mikvah, when husband and wife will be coming together for sexual intimacy after almost two weeks of separation, is considered very special.
Though sexual intimacy between husband and wife is always considered holy in Judaism, sexual activity this night after her visit to the mikvah is considered a special mitzvah, a uniquely holy commandment. In addition, each Friday night is also a special mitzvah for wife and husband to engage in sexual intimacy.
Some say that on Friday nights there are two mitzvoth, commandments, fulfilled though sexual intercourse. The first is the regular mitzvah of sexual intimacy between wife and husband, and the second is the deep pleasure one is supposed to have in honor of the Shabbat itself.