"I will have a party on Friday."
How I understand, you use "ni" when you say you are going somewhere, so you mark the location "to" which you are going . You use "ha/wa" when you add information like "on Tuesday". I might be wrong though.
They translated the sentence all poorly. Think of "ni" as saying "on" or "in". "Ha" indicates the subject of the sentence
を typically means an action is being performed. Think of パーティーをします as "doing a party." Hope this helps, and please correct me if I'm wrong. :)