There are two polar circles, the Arctic Circle and the Antarctic Circle. There are days North of the Arctic Circle, and south of the Antarctic Circle where the sun never sets (summer), or never rises (winter).

There aren't many people living south of the Antarctic Circle, but there are millions living north of the Arctic Circle.

According to Wikipedia:

a day in the rabbinic Hebrew calendar runs from sunset (the start of "the evening") to the next sunset.

Now, how does this work when there are no sunsets?

  • In short, one should remember that the Rabbis have the power of extending the existing Halachah to uncharted areas. Many times there are arguments as each one's imagination is different. THe Sages apparently didn't know about the Earth shape and movement and after the facts became evident, contemporary Rabbis offered their solutions to the problem. For example the calendar doesn't work at poles where there are no hours at all.
    – Al Berko
    Sep 25 '19 at 16:55

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