This week, you are going to buy a new refrigerator (or something else equally big and expensive). You go to the appliance store and find a nice fridge which would be just lovely for your kitchen. You inquire about the price and learn that it is 10 000,- NOK for the unit. You are also told that if you wait until friday, there will be a sale and it will be reduced with 50 NOK, putting the price at 9 950,- NOK.
"Pah," you think, "It's only 50 kroner, that's not much," and you buy it today, not waiting until friday.
Later on, to celebrate your big purchase, you want to buy a new pair of shoes. At the shoe store you find a nice pair, and they are cheap too, costing only 200 kroner. The shoe store too is planning a sale on friday, you are told. On friday, the shoes will be reduced in price to 150 kroner. You think, "That's a good deal," and wait until Friday to buy your shoes.
Why is this? The savings would be the exact same total amount (50 kroner) but when applied to a very expensive item it does not feel as as big a discount as when applied to the shoes. Our brain thinks in percentages in that case. But in doing so it makes us irrational. If you are willing to wait until friday to save 50 kroner on some shoes, you should also be willing to wait until friday to save 50 kroner on a refrigerator. The net savings is the same.
But we don't. Remember the next time you get into that situation yourself: You are being irrational.
Update: Okay, irrational-ish. There are some exceptions and caveats which was kindly mentioned in the comments.