Here we will collect simple principles that any sane and logical man must use while participating in or avoiding big sales days.

Principle 1. All big sales are made to bring profit to seller.

This can be four ways:

Way 1. Previous item price had big margin (both for manufacturer and seller), and during sale they agreed to cut it.
Way 2. Sale is totally fake or near it, original price usually being inflated not long before sale (common for some countries).
Way 3. Sale item being catch to make you buy other items, software, accessories or some service you do not need.
Way 4. Using discount make you buy it via credit. Possible due to owners of big retail giant and bank giant being sometimes same.

Principle 2. Most sellers rely on the impulse sales. Where people do not think if they actually need the item they get.

Solution 1. Make the list of things you really need and add to each item the actual price before sale for few big retailers.
Solution 2. Never buy on impulse. Always rely on logic, but also understand that our brain is very clever in finding logical reasons to buy things we don't need.
Solution 3. If you are in offline store, do not put item in basket or plastic bags right away. Make few long circles around shop instead. As if you put it bag or such your monkey brain try to defend the items with any arguments he could find.

To be continued today and up until Monday.