Many patients call asking this question right before a big event or a vacation. You can definitely prevent a period from coming, but not if you call the week before.
The most successful way to do this is if you are on the birth control pill and you do it a few months in advance. We first figure out when the expected period is, based on where you are now in the pill pack, and if the timing is bad, we count back from the big day and skip the placebo (last week or pill free week) one or more months prior, to get the desired effect. You should manipulate your pill as far away from the big event (do the skipping of the placebo weeks now instead of the month before the event) as some women will get irregular or break through bleeding when they alter their pills. This is not dangerous at all, and you are still totally protected against pregnancy, even if you do get break through bleeding.
If you are not currently on the pill and you want to alter the timing of the cycle, again this should be done as far in advance as possible, to avoid irregular bleeding around the time of your event. The best way is to initiate a birth control pill, with the new start appropriately timed counting back from the week before the event (the week most people want to get it, so they can get it over with before the event). If you don't want to or can't use a birth control pill, then the other option is to use provera or prometrium (progesterone tablets) to bring on your period earlier. Again, this should be done as far from the event as you can.
Note: It is recommended that any manipulation of the birth control pill be done in consultation with your physician.