Where the heroes for the rotation of every week are announced?? and when is it appens?

Check the announcement section of the forums. The weeks champions rotation is posted there (generally on Tuesday iirc), as well as the following weeks free lineup.

Latest announcement is at http://www.leagueoflegends.com/board...d.php?t=126218

Please be aware that the champions that are free in one week are not necessarily going to be free the next week. If you like a champion alot, save up your IP (or purchase RP) to get that champion so you may have him/her forever!

Wich are the heroes that are less probable to appear or will never do in the rotation??

All heroes will appear in rotation at some point. Not sure on probability, but I would expect champions that are harder to play well (like Karthus or Anivia) to show up less than easier Champs (Ashe). That said, I would be surprised if a champion didn't show up at least once every couple of months, Riot does a fairly good job of keeping the free selection fresh.