January 8th 2016 Is My Period Date As Per Previous

January 8th 2016 is my Period date as per previous month. November 2015 i got it on 10th, December on 8th so as that January is supposed to come around 8th only. I wanted to delay my date as there is a function at home on 8th. Started taking Primolut N from Dec 25th and from Jan 4th started using Regestrone with my Doctor Suggestion. But it did not delay my period and i have got on Jan 6th though im using pills. Why pills did not work over me? Im really upset.