Everything you need to know about WoW's 12th Anniversary (World of Warcraft: Legion)

World of Warcraft is celebrating it's 12th Anniversary from November 16th to November 30th in 2016, and this includes a two-week in-game celebration. During this event, all players who log in will get a gift package in their mail box, and it contains the following three items:

1) Celebration Package. This is an item that you can click on to apply a buff to yourself. The buff increases your XP gain and rep gain by +12 % for one hour, but the item can be used as many times as you like, so you pretty much have a permanent +12 % rep/XP buff during these two weeks.

2) 200 x Timewarped Badges. This is a currency that you usually get by doing Timewarped dungeons during the Timewarped event, but it can only be used to purchase some items during the Anniversary. More information on the items is below.

3) Letter from the Timewalkers. This item begins a quest, and is pretty much just there to show you where to find the daily quest and the Timewarped vendor. These are located in Stormwind / Orgrimmar.

All characters get these items in their mail box, at any level, so make sure to pick it up on your alts as well.

Along with the gift package in your mail, there are also a daily quest available during the event, which can be completed once every day. It awards 25 x Timewarped Badge upon completing. The quest is called "A Time to Reflect", and you need to answer one question about WoW history to complete it. Some questions are more difficult than others, but you have unlimited tries, and 4 possible guesses, so for each guess you have 25 % chance to get it right. If you get it wrong, just reopen the chat window to get a new question and another shot at getting it right.

The Anniversary vendor sells a few items for Timewarped Badges, and below are all the items you can buy during the Anniversary:

1) Inflatable Thunderfury, Blessed Blades of the Windseeker (50 Timewarped Badges). Let's you equip an inflatable Thunderfury. It has five charges, and also gives you an achievement.
2) Exquisite Costume Set: "Edwin VanCleef" (200 Timewarped Badges). This items lets you dress up as Edwin VanCleef, and has 50 charges. 
3) Celebration Wand - Gnoll (25 Timewarped Badges). Put a gnoll costume on a party member.
4) Celebration Wand - Murloc (25 Timewarped Badges). Put a murloc costume on a party member.
5) Corgi Pup (200 Timewarped Badges). A battle pet unique to this Anniversary. You can get a total of 3. 

I suggest picking up the Corgi Pup(s) on your alts, because the pets are shared across all characters on the same account. The Celebration Wands can also be sent to other battle.net characters, but the inflatable Thunderfury and Costume Set are bound when picked up.

You can make three level 1 characters to pick up the Corgis if you like, so you don't have to waste the badges on your high level characters. 

If you are saving for one of the Timewarped mounts, you don't have to spend any badges on this event, and can instead get a free 200 Timewarped Badges for using later.