When is the League of Legends champion rotation?

Answer 1

Each week 10 champions are set as "free to play" and you can pick them even if you didn't buy them in the shop. This set is referred as the "weekly rotation". However, if you don't own the champion you won't be able to pick them anymore once the rotation changes until they get in another one.

The champion sale is a bit different concept. Champions can be bought both with Influence Points (IP) which are earned on playing any match of the game (of course, you gain more IP if you win and less if you lose) and Riot Points (RP) which are earned only through payment. Usually, there will be some champion sale with say 50% price reduction for a set amount of time BUT be careful: that discount only refers to the RP price. That is, if a champion costs 6300 IP or 975 RP, when on sale at 50% it will still cost 6300 IP but RP price will be 485 or something like that.

It's useful to remind that there's no content in LoL which can't be bought with IPs else than champions skins, wihch while some are really good-looking, they don't affect gameplay at all.

League Of Legends Champions Guide : elohell.net