What Medication Is Recommended To Delay Period Without Fear Of Allergies?

I am going out for a small vacation from 24th to 28th October. My periods are regular and start exactly in 28 days. I will have my period on 26th. I wish to pre-pone my periods to 18th or 19th Oct. I am 28 years old married lady. I am healthy and not really allergic to any kind of medicine. I want to take the medicine only this time and have never taken it before in life. Please suggest me a medicine and also when to take it so as to have my periods on 18th or 19th. Thanks.