Today is the 23rd of November. 4 years ago World of Warcraft was released in the US, and Blizzard is now celebrating that in-game!
What you will get is an achievment for logging in on this day. You wont get points for this however, since this is a "Feat of Strength". Its cool to have accomplished that though, so log in on all your characters today so that you will get it on them all!
Another thing you will get, is a mail from Blizzard. The mail contains some text from Blizzard, telling you they are grateful that you play their game etc. etc. Most notable however is that you get a pet! A little Baby Blizzard Bear. It is really cute!
You will both get the achievment and the pet on all characters that you login today. So take your time and do it!
Another thing, which is worth mentioning, is that the pet is "Account Bound". What this means is that you can send this pet to your other characters on your account. Might be worthless at start, but when this day is over you wont be able to get this pet anymore. So, for example tomorrow when you create an alt, you can just simply send the pet by ingame mail to the new alt and he will get the pet too!
I realise this might not be a "guide" as you know it, but i think you understand my way of writing nonetheless.
On another note, in Europe it is already the 23rd, so we are able to get it. But due to the different timezones, it isnt the 23rd just yet in the US. So you Americans, wait a while!