Children's Week

Children's Week is celebrated once a year in Orgrimmar, Stormwind, and Shattrath City. The week-long celebration is a time to give back to the innocents of war: the orphans! This year's occurrence of the holiday is accompanied by new achievements, which can be found in the World Events tab of the Achievements menu. Learn more about this special in-game event on our Children's Week information page.

http://www.wow-europe.com/en/info/basics/events/childrensweek/

Q u o t e:
Why I have to play a game that I don't like to complete an achievement that's supposed
to be PvE oriented?

Just to reiterate what others before me have said, world event achievements are not specifically designed with PvE or PvP in mind.

Take the Merrymaker meta achievement for example:
'With a Little Helper from My Friends' requires you to gain 50 honorable kills whilst dressed as a festive gnome. (PvP)

Or the Brewmaster meta:
'Direbrewfest' requires you to kill Coren Direbrew. (PvE)

The Flame Keeper meta:
'Ice the Frost Lord' requires you to kill Ahune. (PvE)

Hallowed Be Thy Name:
'G.N.E.R.D. Rage' requires you kill 50 people under the effects of G.N.E.R.Ds. (PvP)

The achievements are there to provide you with some fun things to do, they are not a requirement. If there is something you wish to achieve in the long run from doing it, then of course you may have to dabble in an element of the game that's not your 'cup of tea', sadly. The PvP Achievements tab contains PvP achievements for those dedicated or interested in PvP. Much like the Raids and Dungeons tab caters for the PvE focused. World Events are not designed to fit in either category but take elements from all the various aspects of the game.