Anyway to find out sales in advance?

Not specific dates, but you can refer to this spreadsheet (or others) which make predictions based on the last time an item was on sale and how the sale rotation works;

https://docs.google.com/spreadsheet/lv?key=0Arh3w_1KfCECdEl3UUUyZ0pFSFBWcmpaTDVfa1lBUGc&toomany=true#gid=13-

Occasionally Riot will announce sales in advance, usually around special events like Snowdown. I think they did a big block of preannounced sales last summer as well.

If you buy something and it goes on sale (unannounced) within a short time, they'll refund you the difference if you send in a support ticket.