List of observances set by the Hebrew calendar
All observances begin at sunset the day prior to the Gregorian date listed unless otherwise noted, and end on nightfall of the date in question, which is defined as the appearance of three stars in the sky. On leap years (which occur every 2–3 years) an extra month, Adar II, is added and certain holidays move accordingly, and it is mentioned in the notes section.
Source: Wikipedia — List of observances set by the Hebrew calendar (CC BY-SA 4.0)