What is the name of the imaginary line drawn on the earth above which the sun will not set for at least one day during the year?

In the northern hemisphere the line is the arctic circle. North of this line the sun does not rise on December 21st each year with more days having the lack of sun the further north you go. The sun does not set on June 20/21st in the same region with more days with full sun the further north you go during the summer.

In the Southern Hemisphere the line is the antarctic circle.The same event takes place just reverse of the regions in the northern hemisphere. ie more sun in the winter and less sun in the summer.