Golden Numbers

This row contains numbers between 1 and 19 called the Golden Numbers, which helped the reader to determine the date of Easter. In this image, they are shown between two purple marks. 

Easter is the most important feast in the Christian calendar, as it commemorates the day of Christ’s Resurrection. It is a moveable feast, meaning it does not have a fixed date and must be calculated every year. 
 
Easter falls on the Sunday after the first Full Moon after the Vernal Equinox. One therefore needs to know the date of the Full Moon. This is where the Golden Numbers come in, as they helped the reader identify the date of the New Moon in any given year. 

To learn more about the calculation, see Seb Falk’s Guide to medieval calendars.