How Can Periods Be Postponed?

Hi doctor.gud mrng.i just wanted to postpone my periods for 5 days because of a religious function at my home.my previous period was on July 10th.but I have been getting my period 2 to 4 days before of my period date.i don t know why it has been happening.my next period is on 10 th august but i have function on 9 th and 12 th if this month.so when doi use tablets for my better convenience.please help.thank u.