This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
A Hebrew-calendar day does not begin at midnight, but at either sunset
or when three medium-sized stars should be visible, depending on the
Sunset marks the start of the 12 night hours, whereas sunrise marks the
start of the 12 day hours. This means that night hours may be longer
or shorter than day hours, depending on the season.