Children's Week
This week is Children's Week. If you've not done it before, it involves a few fairly easy questlines and forms part of the meta-achievement What a Long, Strange Trip It's Been (which awards a very nice looking mount).
If you're a pet collector, this is the one you need .. there are LOADS of pets to get from it. A full list can be seen over on Wowhead.
If you already have all the pets, be sure to pick up Pet Care Package--the reward for repeating the holiday, even if you can't get any pets! Pet Care Package was new to 2015 and contains a variety of items including pug costumes like Big Pink Bow, Magical Pet Biscuit, and Flawless Battle-Stone. This is an alternative reward from the orphan quests and great if you already have the pets!
There are also two toys to collect - one of which is a new toy this year (you could buy it last year, but it wasn't a toy): Yellow Balloon.