Webster's Dictionary 
(1): ( n.pl.) Records or registers of important events.
(2): ( n.pl.) The Roman calendar, which gave the days for festivals, courts, etc., corresponding to a modern almanac.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
the sacred books of the ancient Romans, in which were recorded the fasti dies, or lawful days — days on which legal business might be transacted before the praetor without impiety. These fasti contained a full enumeration of the months and days of the year, the various dates belonging to a calendar, and the several festivals arranged under their appropriate dates. Before the adoption of the, practice of preparing such tables, it was customary for the priests to proclaim the different festivals, for the information of the people.
The Nuttall Encyclopedia 
The name given to days among the Romans on which it was lawful to transact business before the prætor; also the name of books among the Romans containing calendars of times, seasons, and events.