Two Great Announcements made by Riot regarding a Deal on Toxic Mundo skin and the new Champion Rotation Schedule for the following three weeks. First the announcement made by the Associate Media Producer MrCrumb:

For this weekend only, Toxic Mundo charges what he pleases! Starting Friday, save 50% on this green goliath. This deal is only available until January 31st.
Toxic Dr. Mundo skin (previously 520 RP), will be 260 RP!
Click the Skins tab in the Store to purchase and unlock this skin.

Later the Director of Marketing, RiotChris, made the announcement for the Weekly Champion Rotation Schedule,

We have rotated the free Champions for this week. I've also posted the Champions that will be free for the next week (and the ones we are thinking about making free the week after).
I've also slapped a chart at the end for number crunchers and history buffs. We're still reading suggestions for future weeks.

Sources: New Free Champions (Week 10), Toxic Mundo Skin Goes 50% Off This Weekend