From:worldofwarcraft.com

We will be offering free character moves from selected high population realms to lower populated realms, assisting in population dispersal. The ability to move a character on the selected source realms will become available on Friday, January 4 at 12:01 PM PST, and are scheduled to run until Friday, January 11 at 12:01 PM PST.

In the event that a realm meets our transfer goals before the scheduled end date, the transfers to that realm will be disabled. We highly recommend that if you are planning to transfer that you do so at your earliest convenience. No characters will be eligible for these free moves once the transfers have been closed.

Once the transfers open you can begin your transfer by visiting the Character Move page located here: https://www.worldofwarcraft.com/account/character-move.html
The realms chosen consist of source realms with the highest sustained queues, and destination realms with the lowest population. The numbers gathered are directly from our data, and depict the realms in most need of these transfers. We will be continuing to monitor all realms and open additional free transfers in the future where necessary.