There Was Evening, And There Was Morning...
When following the Gregorian (secular) calendar, it is natural to think of the days of the week as Sunday, Monday....Friday, Saturday, each day beginning at midnight and ending at midnight. In the Jewish calendar, however, the names of the days are given as a count toward Shabbat: Day One, Day Two....Day Six, Shabbat, and each day begins and ends at sunset.
In the first chapter of the book of Genesis, the description of each day of creation is noted with the same language: Va'yehee erev va'yehee voker - There was evening and there was morning. It is therefore understood that according to the order of creation, evening precedes morning. Thus, each day begins at sunset. Shabbat and all Jewish holidays, therefore, begin at sunset, the evening before the day of the holiday marked on a secular calendar.
Since the precise time of sunset is difficult to determine (whether sunset means the beginning of the setting of the sun or once the sun has completely set), Shabbat is observed from the beginning of sunset on Friday through the end of sunset on Saturday - a time period that works out to just about 25 hours.