For professional soccer players, and all professional (and collegiate) athletes for that matter, the proper game day preperation on the day of a big game, and a couple days before a big game, taking care of your body properly is essential to getting the best performance on the day of the big game.
Most players will have a light practice (no heavy duty lifting, or running sprints), but for soccer players, going out on the field a few hours before a game and just kicking a few goals and warming up, and stretching. Prior to getting out on the field, a large meal is essential to keep them energized enough for the game.
The proper meal (eaten at least 5 to 6 hours before a game) should be packed with protein, high in fat (but not greasy fat, essential fats), and be packed with calories. The amount of weight which a player losses during a game, it is an essential that they eat the proper nutrients to keep thier body and energy levels up for a game.
The right meal, good nights rest the night before, and a brief warm-up on game day are essential to performing well for a big matchup.