When the sun shines directly, the Tropic of Cancer is a solar term.

When the earth revolves around the sun, the axis of the earth and the orbital plane form a fixed inclination of 66.5, which makes the direct point of the sun's rays move regularly between the tropic of cancer in a year.

When the sun's rays move from the southern hemisphere to the equator, it is the vernal equinox in the northern hemisphere (about March 2 1); When the direct point of the sun is on the tropic of cancer, there will be extreme daylight in the Arctic Circle.

So the answer is: vernal equinox; March 2 1 or so; An extreme day.