Start up your system without the disc inside, then access the ''System Configuration'' option. Choose clock adjustment and set the date to 12/25/2000. After adjusting the date, insert the game and begin. Choose arcade mode and highlight ''Selene Strike'' on the boxer selection screen. The words, ''Holiday Outfit'' will appear underneath her name.
To have the referee look like a leprechaun, set the PS2s internal clock to the 17th March.
To unlock GC Thunder's Uncle Sam costume, set the internal clock inside your PS2 to the 4th July.