The Modern Standard: Midnight
In the Gregorian calendar and modern civil timekeeping, a new day begins at midnight (00:00:00). This is a relatively modern convention — convenient for a 24-hour clock society but essentially arbitrary from an astronomical standpoint.
Sunset: Hebrew, Islamic, and Old Christian
Several ancient traditions begin the day at sunset:
- Hebrew calendar: Shabbat begins at sunset on Friday and ends at nightfall Saturday. Jewish holidays begin the evening before their calendar date: "And there was evening, and there was morning — one day" (Genesis 1:5).
- Islamic calendar: The day begins at sunset, which is why Ramadan nights and Eid celebrations begin the evening before the calendar date.
- Old Christian tradition: Christmas Eve, New Year's Eve, and Halloween (All Hallows' Eve) preserve the pre-modern concept of the evening before as part of the holy day.
Sunrise: Some Hindu and Buddhist Traditions
Some Hindu and traditional Buddhist reckoning begins the day at sunrise. The Panchang tithi (lunar day) can begin at any time, but daybreak is a common practical reference for ritual timing.
Dawn vs. Sunrise
Islamic Fajr prayer marks the beginning of the obligatory fasting period during Ramadan — at astronomical dawn (when the first light appears on the horizon), not sunrise. This distinction matters enormously for communities near the Arctic, where summer dawn and sunrise can be separated by hours.
The Astronomical Day
Astronomers historically began the day at noon, so that a single night observation fell within a single day's date. This Julian Day system (not to be confused with the Julian calendar) is still used in astronomy today — Julian Day 2460000 began at noon UTC on February 24, 2023.
Practical Implications
The definition of when a day begins has real-world consequences:
- Jewish communities worldwide turn off electricity at the moment of sunset every Friday — apps calculate this to the minute for each location
- Prayer timetables in Muslim countries are location-specific and change daily
- Airline ticket "midnight" rules have generated lawsuits over whether 00:00 means the start or end of a date