Like Phukette said. Xmas time and end june. But If you live in holland or US (I think) the sales begin 2 weeks earlier. In other countries like germany, UK, france the sales begin in the last week of december and june. For holland for example it begins first week of december june. Anyway that's what my research told me :p
I also advice to check the little shops in your town, or little webshops... the chance is higher that they do have your size and with greater discount. Diesel give max 30% (US 20%) sale on jeans.
And why people pay full price? Because they have the money, they need a rare size (little sizes).