A luach is a Hebrew calendar. It is one of the most important tools you need to keep the laws of family purity. The purpose of a luach is to guide you in calculating and recording the times one is required to abstain from marital relations in anticipation of a menstrual flow. Learning how to keep your own written calendar and calculate all the times properly is an excellent skill to master, even if you elect to utilize the convenience of one of several, halachically-approved, modern-day online calendars or apps.

​

Below is a video showing a comprehensive review of how to keep your calendar by mikvah.org