Mistake in Counting Seven Clean Days
A lady finds out that for the last number of years she has made a mistake in counting 6 days instead of seven before going to the mikveh. What should be her and her husband’s approach to doing teshuva for this? Have they transgressed a Torah prohibition (even though the seven clean days was originally formulated as a stringency), or:
Is it considered a rabbinical prohibition bearing in mind that she counted 5 days after her menstrual period and carried out the correct bedikos throughout.
The seven clean days are the Biblically prescribed method for attaining purity for a woman who has the status of a Zava Gedola, a women who bled for 3 days beyond the 7 days of a regular nidda. Chazal instituted that every nidda, i.e. every women that becomes a nidda even from slight bleeding or staining or a regular period should observe the 7 clean days. This was to avoid confusion and not violate nidda when a woman in fact needs 7 clean days Biblically.
So while they were probably technically only in violation of a Rabbinic law, this law is treated with utmost stringency as it is a safeguard from violating a very severe prohibition which carries with it kares as a punishment. There are many authorities who will not allow leniency in the seven clean days even if it means the couple may be forced to separate!
In earlier sources we find procedures for atonement of this prohibition, which includes many fast and other such methods. In our day and age, leading authorities do not recommend these methods as we are no longer capable of doing them properly. Rather, the couple should learn together a set time every day the laws of Nidda and give a sizable donation to charity, perhaps to the local mikva or to promote Jewish education in these matters. If the opportunity arises to teach someone unaffiliated the laws of nidda, that would be a great step.