Suggest Medication To Delay Or Prepone Periods

I m going on a beach trip on Friday (August 8), and I m worried I might get my period then. It s been weird lately: starting points have been April 27, May 27, June 20, July 10. This summer I ve been at a boarding school and so stress might be a factor in shorter cycles. But my question is: is there anything I can do to make my period come either tomorrow, August 3, or August 11? Anything to miss this weekend? It usually lasts four days.