The Answer:

We're not really sure what you'd consider a practical answer. Beginning a day at nightfall—as the Jewish and Islamic calendars do—is no more or less arbitrary than beginning it at midnight or sunrise.

The Jewish practice of beginning and ending every day—holidays included—at sunset is established at the very start of the Bible. Genesis 1:5 says "it was evening and it was morning, one day," with the evening being the start of the day. This pattern is continued for the remaining days of creation through to "it was evening and it was morning, the sixth day" in Genesis 1:31.

—The Fact Monster