At any place between the equator and 23.5 degrees south latitude, there are two
longest days of the year. One is a day between September 21 and December 21,
depending on the latitude, and the other one is the same number of days after
December 21 that the first one is before it. The days are of equal length.
At any place between 23.5 and 66.5 degrees south latitude, the longest day is December 21.
At any place farther than 66.5 degrees south latitude, the longest day is somewhere
between 24 hours and six months long, depending on the latitude, so it hits more than
one date. December 21 is the middle of it.