Trivia: It's not an accident that Wisconsin was chosen as the new South Pole in the movie, its position being 44N/90W, 44 being a number that Roland Emmerich tries to sneak into every movie he makes since his 'Moon 44' days. (01:37:25)
Trivia: "Farewell Atlantis" was the fake working title of the film, the same name as the book written by Jackson Curtis in the story.
Trivia: Though named differently, all the state leaders (plus the governor of California) shown in the movie are of the same gender, ethnicity and general appearance as their real-world counterparts who ruled in 2009.
Trivia: It isn't adequately discussed, but if the new South Pole is in the middle of Wisconsin (44N/90W), then the new North Pole is in the neighborhood of St.Paul island(44S/90E), in the southern Indian Ocean. One can only assume they are referring to the magnetic poles, not the axis that the earth rotates around, otherwise Africa would be as hospitable as northern Canada.
Answer: Because it's the 21st December in the MAYAN calendar. In the calendar most people use, it was actually July.