Skins can be purchased or gifted this month, December 2013, February 2014 and June 2014. The decision to re-release these skins was part of a year-long exploration on how "to solve a conundrum" within the community. According to Riot, the most common skin, Bewitching Nidalee, is seen only once in every 971 games. The least common, Ice Toboggan Corki, is seen once every 116,586 games.
"During events from December 2009 through October 2011, we released 34 skins for a limited time and communicated they wouldn't come back into the store," the post reads. "Since then, our playerbase has grown so much that these skins are virtually never seen in game.
"Every day we receive heartfelt emails with compelling stories from players asking for the chance to own or gift their favorite skin, either because they missed the brief store window due to travel, illness, deployment, etc., or because they hadn't joined the LoL community yet."
As compensation for the "reduction in exclusivity" for those that already have the skins, Riot will give players RP equal to the price they paid, an exclusive icon and vintage treatment in their loading screen when the skin is used.
For a list of skins and their future availability, check out the full post.