WoWDB

Hallow's End is Azeroth's version of Halloween. The very first Hallow's End took place in 2005, but the Headless Horseman (holiday boss) was not added until 2007.

At 4:10pm, 8:10pm, and midnight PDT (GMT-7) - not local realm time - there are role-playing events by Lady Sylvanas Windrunner and King Genn Greymane at the Wickermans at their respective locations in the Ruins of Lordareron and outside Stormwind's gates. Horde players present for the Forsaken's event receive  Decree of the Dark Lady (level 1-79) or  Decree of the Dark Lady (level 80+). Alliance players present for the Gilneas worgens' event receive  Greymane's Resolve (level 1-79) or  Greymane's Resolve (level 80+).

The holiday has always been good for alts - looting candybuckets awards experience and gold - but with the 2011 overhaul, it is even better! Wickerman Ashes in the Ruins of Lordaeron applies a 2-hour character buff that increases experience and reputation gains by 10%! (  Grim Visage (H) /  Unburdened (A)) You also can buy masks, wands, pets, the holiday-only mount (the broom) using  Tricky Treats. The masks also are now applied as a player buff instead of being equipped in the head slot.

• Horde vendors: Darla, Chub, Farina.
• Alliance vendors: Stymie, Dorothy, Spanky.

Players can earn  Loot-Filled Pumpkin once per day per character. You can queue for the holiday boss via LFD or by speaking to the Headless Ghost in capital cities and some sanctuary cities.

For the "Tricks and Treats..." series of achievements, I recommend installing the add-on Candy Buckets (Hallow's End). It adds small candy bucket icons to your world map, and even knows and remembers buckets you've visited! It has TomTom (and TomTomLite for those users) integration, too. Spiffy!

For a thorough guide to the world event, read Icy-Vein's guide.

Patch 8.0.1 added the toy  Headless Horseman's Hearthstone for 2018.  The related armor and weapons were also updated -- post-item level squish -- to item level 335 at level 120.