During the Hallows End festival you get a chance to face the Headless Horseman in two forms.
The first is in most newbie towns. The Horseman arrives at Four, Eight, and Twelve server time. He sets buildings on fire and it is your job to grab water buckets and to throw them on the fire. If you target a person and throw the bucket at them it will appear in their inventory. With some strategy you can form bucket brigades to put out the fires faster. If there is not enough people to put out the fires within the allotted time then the Horseman wins and you get nothing. Once the fires are out the Horseman, which is level 11, comes down and you kill him. The first person to hit does not get to loot him. A burning pumpkin will appear with many goodies including a rickety broom which any level can ride but has no speed increase.
The second way you can face the Horseman is to go to the Scarlet Monastery Graveyard. This version is a level 70 elite. You can go in with a raid party of 10 people. Anyone level 66+ can get the quest to summon him. To kill him you must fight him and when he is almost dead his head appears and you have to fight that. Once his head is dead it goes back to his body and you need to fight back and forth a few times. After the 2nd time he will spawn 5 level 70 adds. Do not fight the adds unless it is for CC. They will disappear once he is dead. A raid party can repeat this over and over until all the level 66+ members have done the quest. Once the Horseman is dead run outside the instance and have the raid leader restart the instance. You are limited to 5 instance runs an hour so you may need to wait a little while if you run through them fast.
So long as the fighting group is strong enough you can run lower levels through the instance. This is a daily quest so everyone level 66+ can run this every day during Hallows End. There are many purple items that drop, 4 brooms, and a squashling non-combat pet. Thottbot
There is 2 ground mounts, level 40 and level 60, and 2 flying mounts, normal and swift. You must meet the riding skill requirement to use them. The 14 day duration means 14 days of played time. You can collect mounts and send them to an alt you never play and save the mount for later in the year. As long as you are logged on the toon that has the mount you are running down the timer.
Make a macro to auto target the Horsemans head by typing /target Head of the Horseman You may need to spam click the macro.
Before the adds become targetable have hunters lay down frost traps and have mages ready to Frost Nova the adds.