Bible Encyclopedias
Nundines or Nundinal Letters.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature

The Romans used letters called literae nundinales, eight in number, to denote the dies profesti, nundinae, in their calendars. The nundins, or market-days, happened every ninth day. In imitation of them, the European nations have adopted seven dominical or Sunday letters, one of which denotes the Sunday throughout all the months of the year. (See DOMINICAL LETTER).