You needed to complete all the Midyear Mayhem achievements.
To check progress you press start
, then "JOURNAL", "ACHIEVEMENTS" and then "HOLIDAY EVENTS".
To participate in this event you'll need both the first expansion Imperial City and the newest one for Morrowind.
Echo of Pelinal's Fury - Read a Scroll of Peinal's Ferocity during the Midyear Mayhem.
(This can be done right after finishing the free quest that starts the event.)
Hand of the Whitestrake - Acquire your 25th "Pelinal's Midyear Boon Bo".
(You get two other achievements that get superseded for getting your first and tenth box, you need to get 25 all together.)
You get your first box from the starting free quest but then subsequent boxes you can get just by completing Battlegrounds matches via the Activity Finder. You don't have to win, just stay till the match is completed, then check your mail.
An even better way to get boxes is to find a group in Cyrodil and just mess things up while you do billboard quests. Each quest completed in Cyrodil gets you an automatic box. Attacking other players and places will get you Alliance Points (AP) and after every 20,000 AP you will get a box in your mail. Don't forget to activate the scroll from the first quest every three hours to get bonus AP.
Mayhem Connoisseur - Receive the blessing of the Whitestrake at one of the Alliance Gates during the Midyear Mayhem.
(This is for completing the free quest that starts the event.)
Pelinaline the Bloody - Capture a Keep in Cyrodil during the Midyear Mayhem.
Keeps are on the outer ring of Cyrodil. Join a group and just follow them around, they'll probably attack one at one point.
Pelinavant the Scourge - Win a Battleground match during the Midyear Mayhem.
(This was originally broken but was fixed after a couple days.)
Que up for a Battleground match via Activity Finder, just win once.
Pelinerrif Insurgent - Capture an Imperial District during the Midyear Mayhem.
The Imperial City is located in the center of Cyrodil. You'll need to kill three AI minions guarding the flag area of the district and then simply stand by the flag to capture it. Watch out for real enemy players.
Wrath of the Whitestrake - Defeat 50 opponents during the Midyear Mayhem.
You can get this pretty much anywhere and you'll probably get a good amount of kills doing all the above.