Lest you think this is an entire free month for the game, there are more restrictions in place. Former players can log in at any point during the campaign and enjoy four days’ worth of access to the game, meaning that it’s not tied to a single four-day period but still lasts for just four days. Accounts used to log in also need to have been inactive for at least 30 days before being eligible. So you can’t hop in if your subscription just ran out, but if you’ve been away from the game for a little while you could take a step back in to say hello.
Final Fantasy XIV but still want to get in on the additions from patch 3.1, you have a chance with the game’s new free login campaign. While previous opportunities to log in for free after a patch were generally restricted to a single weekend, the new promotion is offering players nothing but time, as the campaign is running from December 1st to December 31st.
Source: Official Site; thanks to Darkstar for the tip!