YOU SAID:
i am going to the birthday party of my best friend
INTO JAPANESE
私は私の親友の誕生日パーティーに行きます
BACK INTO ENGLISH
i'm going to my best friend's birthday party
INTO JAPANESE
私は私の親友の誕生日パーティーに行きます
BACK INTO ENGLISH
i'm going to my best friend's birthday party
That didn't even make that much sense in English.