Here is RiotLuqi, a Brazilian community specialist, with the details:
"Starting today, players'll be able transfer to the Brazil server. This transfer is free for 2 weeks from Friday May 17th to Friday May 31st at 8:00pm PST. Use your free transfer token by heading to the in-game store and click on the Other tab. Secure your summoner name on the Brazil server by transferring today!
For the full scoop on the Portuguese-language server, transfer options and more, check out this FAQ."