At the Character select screen press:

LEFT, LEFT, RIGHT, R2, LEFT, RIGHT, RIGHT, R1, R1, R2

All characters will be unlocked. The question mark in the lower-right corner is the "Random fighter" button. Before you press X to confirm the fighter you want you can press the SQUARE button to change their costumes. You will have access to every costume, including the Holiday outfits. Not every boxer has a Holiday Outfit.

Note: This code also works in Championship mode.