The aim of this study was to analyse the importance of the order and characterization of the days (Fasti) for the legislation and jurisdiction in ancient Roman civil law (ius civile). After having summarized the order of the months and weeks in the Roman lunisolar calendar, both the categories dies fasti / dies nefasti (according to Varro: „dies fasti, per quos praetoribus omnia verba sine piaculo licet fari”) and those of dies comitiales and dies nefasti publici were to be defined. Finally (as the adjective fastus can be derived from the same root as fatum, i.e. the verb fari), it seemed to be worth throwing some light on the sacral roots of the legis actio sacramento.