בראשית, פרק א׳, פסוק כ״ג

The rhythm of time established at the dawn of creation continues to shape the daily and spiritual life of the Jewish people. Rather than measuring a day from the moment the sun rises, the biblical concept of a full twenty-four-hour cycle operates differently. [חומש קה ת; ביאורי חסידות] notes that a day actually begins with the onset of nightfall. The cycle then continues through the darkness and the subsequent daylight, concluding only when the next night arrives.

This foundational pattern, set in motion during the initial days of creation, serves as the basis for the entire calendar. It is the reason why the Sabbath and all other festivals are observed starting from the evening before the calendar day itself, rooting ongoing religious practice directly in the original structure of the universe.