Why is Shabbat 25 hours long instead of 24?

Shabbat lasts from nightfall to nightfall—as does every Jewish calendar date. But when exactly is nightfall? When the sun sets or when darkness sets in? Well, this is an age-old question, a halachic grey area. So we cover all our bases just to be safe. We begin observing a calendar date before sunset and it lasts until after the stars appear the following night. (For more on this topic, see Days.)

Additionally, there is a mitzvah to "add from the profane to the holy." In simple words, this means that we celebrate Shabbat and Jewish holidays for at least a bit longer than the prescribed time, to show how precious these days are to us.

Yours truly,

Rabbi Menachem Posner