Records maintained by the early Christian churches and monasteries for the purposes of calculating the date on which Easter fell. The rule, roughly speaking, being that,
Easter Sunday is the first Sunday after the first full moon that occurs after the 21st of March. If this full moon happens to be on a Sunday, then Easter Sunday is on the following Sunday.
(See omegas write-up under Easter for details of the algorithm derived by Gauss for the exact calculation.)
These tables tended to be kept for some years afterwards and clerics would often add notes as marginalia recording significant events of that year. These marginalia were sometimes later extracted and used to compile annalistic documents such as the Annales Cambriae