The border key is the ortho-octonian Easter key. It is used to read the Easter table quickly: the letter indicates the date on which the Easter is celebrated. The key is next to the Sun circle, the Moon circle and the date of Easter. For example, the year key for 2025 is P. This means that Easter falls on April 7th, that is, April 20th, 2025.
This is a service mark that helps to compare the year with Easter.Border keys tableKeyChristmas date in the old styleOrder number in the Easter rangeA22 March1B23 March2B24 March3G25 March4D26 March5E27 March6J28 March7B29 March8Z30 March9I31 March10I1 April11K2 April12L3 April13M4 April14N5 April15O6 April16P7 April17R8 April18S9 April19T10 April20U1121 AprilF12 April22H13 April2314 April24C15 April25 April16 April26March17 April27 April27 April28 April28 Easter19 April29 April2021 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 April23 If the key is on the old style block, the key must be read on the calendar.
The date on the new style depends on the difference between the Julian and Gregorian calendars. In 19002099 the date on the old style is added 13 days, but after 2100 the difference increases. Therefore the site separately calculates and shows the Easter of Christ on the old and new style.