I was on birth control for a while and had been skipping my period. Last month I went off birth control and got a very light period tht ended before the fifth day. On the fifth day i did a hefsek tahara but my husbadn and I miscalculated the 7 days and I went to the mikvah a day early. What should we do we are freaking out because it is an issue of chayav karat. ]Thank you.
While teshuva is certainly in place, “freaking out” is not necessary. The main teshuva you can do is set in place a system which provides an extra assurance that this mistake will not happen again. Aside from this perhaps reviewing together the laws of Niddah would be an appropriate measure. Another possibility would be making a donation to your local mikvah.
Keep in mind that no one is perfect and while this is a very serious matter, you probably did not violate and actual Karet prohibition.