Easy now, we're not talking this year. It happened in 1925, Ethel McGary became the first woman inside 25mins over 1500m on December 31, and given that she did so at Coral Gables, Florida, this is one record that may well stand the test of time (perhaps a meet in California sometime...).

McGary never made it to the Olympics at a time when 400m was the full stretch for the woman in a man's world: she held three world records over 880 yards (in 110-yard pools) and 1,500m, as well as several other imperial distances in between and just beyond. The first woman inside 13mins over 800m (and 880y), with a 12.58.2 effort in Detroit on August 6, 1925, McGary concluded a great year with a 24.07.6 swim over 1,500m at Coral Gables that wiped 59.0sec off the world mark held by fellow American Helen Wainright. Those were the days!

The three closest to McGary's record dateline are:

Dec 28, 1966 - Catherine Ball (USA): at the 1st anniversary meet for the International Hall of Fame in Fort Lauderdale, Florida, Catie Ball clocked a 1:15.6 over 100m breaststroke to shock a field that included four Olympians and shave 0.1sec off the world record that had been held for five months by the great Galina Prozumenschikova (USSR) to start a run of five world records over 100m breaststroke. Ball won a gold medal as a member of the 4x100m medley relay at the 1968 Games in Mexico but then succumbed to a viral infection, lost 5kg in weight and finished 5th in the 100m, just 0.6sec outside a medal. She missed the 200m altogether, the crown won in a time 5.9sec outside her world record, part of that slowdown explained by the high altitude of the Games. Ball, deprived by illness of a clear shot at two solo golds, was still world record holder (long after she had retired) as the class of 1972 lined up (and got out) at the Munich Games and again at the inaugural 1973 world championships in Belgrade, her 200m standard broken the following year.
Dec 20, 1987 - Janet Evans (USA): in 4:05.45 over 400m freestyle in Orlando, Florida (obviously the place to be in December), the distance free legend-to-be ended Australian Tracey Wickham's 9-year-reign at the top of the pile. And we all know what happened next, I assume.
Dec 20, 1949 - Leonid Meshkov (USSR): in 1:07.2 in a 25m pool in Moscow, the Soviet swimmer clocked the first of five successive records over 100m breaststroke-butterfly (for those who look on confused: there was a time when breaststroke rules had a loophole that allowed swimmers to use a breaststroke kick with a butterfly overarm action)

And from the list of world bests in events that turned into world-record distances in 1998:

Dec 22, 1982 - Dirk Richter (GDR): in 26.44 over 50m backstroke in East Berlin
Dec 22, 1982 - Ina Kleber (GDR): at the same meet in East Berlin, a 29.67 over 50m backstroke
Dec 22, 1987 - Birte Weigang (GDR): at the same East Berlin meet five years on, a 27.75 over 50m butterfly.

It would be remiss of me not to remind readers that the German Democratic Republic operated a state doping programme called State Plan 14:25, which saw doctors and coaches administer steroids to an estimated 10,000 athletes over a 20-year period up to the fall of the Berlin Wall in 1989. That not only caused serious harm and health problems for many of the athletes caught in the sporting crime of the century but robbed many an athlete of their rightful place in history.