Is it Haram (forbidden) to have Mehndi/henna party?
I cannot say it is Haram because Haram is something that needs to be based on Nass (evidence expressed clearly in the Quran or Sunnah). I would say it is cultural, it is okay. As long as it fits in general restrictions, observes Islamic rules and regulations in general terms, I would say it is okay for sisters to have a celebration to prepare the bride for this big responsibility she is getting ready for. Henna is allowed.