Anyone know when runepages come on sale?

I remember them going on sale 2 times a year at a 2 for 1 discount, but I don't remember when. Just trying to decide if I can afford another champion and still have time to save up the 6300 IP.

I know Riot mentioned the times they did it about a year ago but I couldn't find the page. If anyone remembers let me know. Thanks, and have a wonderful day!