"Why aren't you going to the festival?" - Bakit hindi ka pupunta sa festival? "Why didn't you go to the festival?" - Bakit hindi ka pumunta sa festival? You can use the word festival since there are a lot of English words used in Filipino; festival is one of them. And actually, you are right calling it Filipino, not Tagalog. Our national language is actually called Filipino, not Tagalog, but a lot of foreigners and even Filipinos call it Tagalog.

" why aren't you to the festival?" (maybe "why aren't you at the festival") >> Bakit hindi ka nasa pista? "why didn't you go to the festival" >>Bakit hindi ka pumunta sa pista?

Your question should be this : ..."Why aren't you 'at' the festival?" ...? The Tagalog term for festival here is piyesta, derived from fiesta.

lol i mean Tagalog LMAO