On race day, it is necessary to be as strong as possible. You want to be able to maximize all the workouts from the previous months. The goal is to push your boundaries beyond your day to day workouts. One technique for doing this is to use the concept of tapering. Tapering is a period of time that allows the body to recover fully from all the hard work, balanced with workouts for keeping sharp. It is a way to maximize the full strength of your body.

The general rule of thumb is the more important the race, the more time the taper will last. Using 10k track runners as an example, during the beginning of the season they may be racing at least once a week. Other than the day before the race where they run a relaxed run they may not do any tapering. They will continue to do full workouts for the rest of the week. During mid-season, they may reduce the mileage or take Additional days off for an important event. Near the end of the season when the most prestigious races occur, they will start eliminating all hard workouts and reducing mileage in the two weeks prior to the big events.

The concept is the same for marathoners. As the race approaches, it is necessary to back off in the intensity and reduce overall mileage. Assuming this marathon is the peak of the season it is best to start tapering two weeks prior to the race. If there are any light injuries that are not fully healed, or your legs are just feeling dead, then add another week to the taper.

Two weeks prior to the race, there will a small reduction in the mileage and the intensity of the hard workouts will dramatically decrease. For advance runners, there is a delicate balance between freshening the legs and keeping sharp. In the week prior to the race the mileage is now reduced dramatically. For some, this reduction should be at least 50% of your total mileage. The intensity of the hard workouts is all but eliminated.

There is an emotional component to tapering that should be addressed. With a strong reduction in the overall intensity of the workouts, the body will essentially go through withdrawals. It is extremely easy to find yourself getting edgy. As the race approaches, one can find themselves bouncing off the walls in the proverbial sense. This is also because adrenaline is starting to go through the body in preparation of the event. It is necessary to acknowledge this is a component of racing that one just has to ride through. This is actually a healthy thing as the body is preparing for the battle ahead. There are some truly primal aspects that are helping the body prepare for the event. This is undoubtedly the time to practice going with the flow. It is best to try mitigate all potential land mines in advance. Give yourself more time than normal for everything, whether it is going to the airport, the Pre-race expo, a restaurant or the start of the race. The object is to preserve as much as that pent up energy as possible.

Sharpening.

For the advance runners, there is another component to the tapering phase and that is called sharpening. Without sharpening, some runners will feel out of rhythm on the day of the race after a strong 2 weeks of tapering.
One way to combat this is to incorporate some very focused running in small portions of selected runs in the last couple of weeks leading up to the event. This will help one maintain both mental and physical sharpness. For example, in a ten mile run,  2 miles of the run may be at a focused pace well above race pace. It is important to shut down the pace before getting fatigued. Another example for the people that run track workouts is to scale the distance and the intensity down. It is till important to get up to speed. One should be very focused but at the same time relaxed.
This is counter intuitive for many, but I like to get a solid, (not hard) workout the day before. Just hard enough to give notice that it is time to run hard soon.
I also think it is important not to get to warped up in the expo or others way of needlessly spending your pent up energies such as exploring a new town or spending extra time going out with family and friends. Save this energy for the race

 The run the day before the race.

The general school of thought is to take this day off.  On a personal level, I have found  taking off the prior day to the day before the race and putting a solid run on the day before the race  provides more benefit. This run should be slightly harder than an easy run but short of a run that will fatigue the legs.  It is very important to have both a good warmup and a good cool down. This run will help shake out the cobwebs of the previous two week taper.