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?