The current ecclesial legislation recognizes two differing concepts: canonical day (c. 202 §1) and liturgical day. How did the Church come to differentiate between these two and what are the exact differences? The first two parts of this article attempt to answer these queries by tracing the historical development of the termini a quo and ad quem for the canssue onical and liturgical day respectively. The article examines the historical and legislative evidence dispersed throughout the centuries, commencing with the Early Church. The final part of the article considers the concurrence of two Mass obligations (Sunday and a holy day of obligation) during the same evening. Can one fulfill these two obligations by participating in one Mass only? Using this concrete example, the article analyzes canon 1248 §1 and the principles and theories used to determine the application of the respective definitions of the day and the resolution of possible conflicts.