Here's Hippalus with more info:
"Here at Riot Games we know the value of a good book – especially if that book happens to be loaded with powerful arcane runes. So to help you get the most out of your seals, marks, glyphs and quintessences, we're holding a sale on Rune Pages.
Between May 23rd and May 30th at 11:59 PM PST you'll receive two rune pages for the price of one on purchases made with either RP or IP.
You'll find the 2 for 1 Rune Pages item in the Bundles tab of the League of Legends store, so don't miss this chance to bolster your book while they're on sale.
Please note: this is a pre-announced promotion, meaning that we won’t be offering partial refunds on rune pages bought between now and the sale. Please mark your calendars!"
The dates vary slightly between regions, check your servers post for more info!