When is the best time to buy an iPhone 5s on sale?

After Christmas? Black Friday? New Years? When will be the best time?

At new years coz you can buy it in discount and some other offers too

When the iPhone 6 is released.

They are NEVER on sale. Apple doesn't allow it. Maybe next year when the iPhone 6 comes out, they'll be on sale, but not until then. You might be able to find someone selling one on eBay or selling a refurbished one for cheaper. But if you're waiting for some kind of sale, you'll be waiting a long time.