Or is the skin sales always announced in Riot's news page?
The news page I'm talking about: http://na.leagueoflegends.com/en/news
Cause I heard someone say that you should check the store cause there may be random skin sales without announcing. Is that true?
Not anymore. They're all announced a month ahead, and skins go on sale every Monday and Friday. The actual date of when a particular skin goes on sale in a month is not given, so that's probably why you were told to check in the store.
Tuesday and Friday are the day of new sales. All sales are announced on Surrender at 20 ahead of time.
