Why do Jewish holidays begin at sunset the night before?
In many modern cultures and calendars, days begin in the morning and end at night.  But what if the opposite were true?   In the opening lines of the Creation story in the book of Genesis, the foundational story for all Abrahamic cultures, we learn that all the world was darkness and void.  (Darkness came first.) And then God said "Let there be light."  (Light came second.)  "And there was evening, and there was morning—the first day." 

This last sentence is critical. The first day consists of evening followed by morning. The Jewish calendar abides by this tradition.  Its days are not morning to morning. Its days are night to night (as marked by sundown).  

As an example, the Jewish sabbath (Shabbat) is on the 7th day of the week. Shabbat = Shevat in Hebrew, which literally translates to the number seven.  The seventh day of the  week is is Saturday. However, Shabbat doesn't span Saturday morning to Sunday morning. It spans Friday night to Saturday night.  Many observant Jews will welcome the Shabbat with a family meal, candles, and prayer on Friday nights.