Bloodborne's Halloween event is fully organized by the Bloodborne fan community. It's an unofficial official event (just go with it), and rules are determined on the Bloodborne sub-reddit based on which suggestions get the most upvotes.
Currently, the rules include choosing and creating a specific costume, like the Grim Reaper or Masked Hoonter, and using the invasion and resonance bells as often as possible.
The event started on October 5 at midnight and runs all the way through November 5. This is still very much an event in the making, so it's bound to change as more rules develop.