A week ago Riot said it'll be coming in "the next few months". That's the most specific time frame I could find.

Thanks to Fatelblade for the sig!
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Source by Bubobubo
TL;DR: Riot is updating how their sales work for the first few months of the new year.
In 2019, Riot will be changing up how they handle sales on skins and other content. They'll be focusing on more favorites like Your Shop, event passes, and the Essence Emporium. They’ll also be experimenting with weekly sales over the next few months.
Overall, the total amount of content going on sale will go up. You’ll see the balance shift towards newer and higher quality skins, but Riot will still look to keep a healthy presence of champions and classics in the mix. There will also be some weeks that look pretty different from what you’ve seen before.
Some features of the new weekly sales they’re testing:
What do you think of Riot's new strategy regarding champion and skin sales in 2019? Leave your thoughts in the comments!