When border between the worlds is thinner, ghosts visit the Azeroth looking for a proof of the living ones' remembrance.
Enjoying the return of the lost friends, it's rightful and worth to honour them!
Prepare at least one Simple Flour and Ice Cold Milk to bake a piece of Bread of the Dead for the revered spirit.
Acquire some Orange Marigolds or a bouquet of them (and recipe for the bread, if you haven't purchased it before)
from Chapman in either your racial capital*, Shattrath City or Dalaran.
Then -- standing near braziers of blue Ghostly Fire -- create holiday food from reagents.
After that, use your marigolds to see a spirit with quest for you. They ask you for the prepared bread, so give them a piece.
When their request is fulfilled, they give you a nice event-themed pet -- Macabre Marionette (item).
Quest can be done once per character, per year.
So even if you have done it the previous year, don't hestitate to see the ghosts again!
They still need some love. ^^
* Worgen go to Darnassus, gnomes to Ironforge, trolls and goblins to Orgrimmar. Pandaren can check only one of the sanctuary cities.