What day of the year is the short one?

Answer 1

If by day you mean the opposite of night, i.e. the time between sunrise and sunset, it's the day of the winter solstice, which is between June 19 and June 23 inclusive in the Southern Hemisphere and between December 20 and December 23 inclusive in the northern hemisphere.

If by day you mean the time from midnight to midnight, it's the day on which daylight saving time or summer time begins. In countries that do not observe daylight saving time or summer time, every day is 24 hrs. long by the clock (not counting leap seconds).