Shabbat Ends at Dark

You may not end Shabbat before dark (appearance of three medium-size stars--tzeit ha'kochavim).
When Is Dark
Shabbat (and Jewish festivals) ends at “dark”: when three medium-sized stars are visible overhead.
Note To find medium-sized stars, look for stars to appear in the west—those will be large stars. When large stars appear in the west, medium-sized stars should be visible overhead.
If you are in a place where the sun sets but the sky will not get dark any time that evening, ask a rabbi what to do.