The reason Jewish holidays and celebrations begin in the evening is rooted in the biblical story of Creation, which teaches that God created night and then day; night came before day.
On the Jewish calendar, holidays begin in the evening, at sundown, and they continue through the next day. For example, the weekly holiday of Shabbat (the Jewish Sabbath) begins at sundown on Friday night and ends at sundown on Saturday night.
For many Jewish families, it is customary to begin the holiday just a bit before sundown and to extend the holiday a bit after sundown the following night.