In article the urgent problems are considered in the field of Historical Chronology and Source Studies at the present stage of development of these branches of a historical science. The author addresses to such valuable historical source as a ecclesiastical calendars. It should be mentioned that in Ancient Rus' every church had its own ecclesiastical calendars. That is why there were a lot of calendars, which were used in that period of time in Rus'. But ecclesiastical calendars did not have only church meaning; they were used also as civil calendars. While re-writing the calendars and chronicles some changes were introduced and that's why some displacement of dates took place there.

The careful research of ecclesiastical calendars will help with specification dating. Research ecclesiastical calendars till now were not developed of a method. The given work is the first attempt in this area. In clause the basic stages of this work are reflected and some preliminary conclusions are made.