If you're only going to one park I'd choose one that didn't have Extra Magic Hours for that day. If it were me and I wanted to do Magic Kingdom I'd go on Friday. On Saturday I'd go to Epcot and Sunday MGM. Magic Kingdom has a tendency to be busy on most Saturdays. The rest is just avoiding EMH days. On a side note, the parks have different hours with Epcot usually closing the latest (9:00). MK will be open til 9:00 on Saturdays but personally I'd stick with Friday at the Magic Kingdom. MGM usually closes at 7:00 and Animal Kingdom at 5:00.
Here's the latest on EMH from
www.disneyworld.com:
January 2006
Morning Extra Magic Hours
Magic Kingdom® Park - January 5, 12, 19, 26
Epcot® - January 3, 10, 17, 24, 31
Disney-MGM Studios - January 7, 14, 21, 28
Disney's Animal Kingdom® Theme Park - January 2, 9, 16, 23, 30
Disney's Blizzard Beach® Water Park - January 1 - 31
Evening Extra Magic Hours
Magic Kingdom® Park - January 1, 8, 15, 22, 29
Epcot® - January 6, 13, 20, 27
Disney-MGM Studios - January 2, 9, 17, 24, 30
Disney's Animal Kingdom® Theme Park - January 4, 11, 18, 25
Have fun!