Don’t work out later than three days before your event. If your party’s on Saturday, for example, do your last workout of the week on Wednesday—muscles bulge most when they’re flush with glycogen (the carbohydrates stored in muscle), so you need to give your body time to recover and replenish its glycogen stores before your unveiling. If you train too close to the big day, the carbs stored in your muscles will be depleted, and you’ll end up looking flat and soft. Furthermore, make sure this last workout focuses on the areas you want to highlight, such as chest and arms. Glycogen is replenished fastest in muscles that need it most—i.e., the ones that have just been worked—so if you want your pecs and biceps to pop, you have to make sure they soak up as many of the carbs you’re taking in as possible (explained in the next step).
There’s one caveat, though: Don’t train your abs. (You’re welcome.) While you need to work abs most weeks, the point of workouts at this stage is to get the muscles that pump up well to swell further with increased glycogen storage. Since abs don’t get pumped like biceps or shoulders, it doesn’t make sense to drive glycogen into them—in fact, doing so can cause them to inflate too much, blurring definition.
Also, don’t do any cardio. (Again, you’re welcome.) Because you want to maximize glycogen, interval training—which uses stored carbs for fuel—would be counterproductive. You can do some light walking or other aerobic training if it helps you keep your sanity, but nothing that could deplete your energy. Keep it to under an hour and perform it at a very low intensity.