From:worldofwarcraft.com

We will be offering free character moves from select high population realms to low population realms beginning December 6, at 12:01 PM PST.

For more information on what realms will be eligible for these free moves and exact dates please visit the announcement post made here.

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.