All stores run sales and the same is for supermarkets. They have new product lines coming, there may be some products that are not selling well and other reasons why they want to reduce prices to get rid of certain products.
So, the very easy general rule is: Buy when they have sales. To me it seems obvious, but I’ve realised that it’s not so, not everybody does it.
This means, for example: after Easter for chocolate, after Christmas for Christmas cakes (you buy one for Christmas and you can have the rest at breakfast all other days). Anyway, they will always have sales, Buy 1 get 1 free, Buy 1 get 1 half price and similar. So don’t worry if you miss something, there will be something similar soon.
Also, after a bank holiday, they may have a surplus of perishable goods (fresh food, like fruit, vegetables, meat, fsh, cheese, flowers), because they had less time to sell their stock, so keep an eye on those and go shopping then.
Big ones like Tesco, Sainsbury’s (and equivalent in the world), often allocate a special area where during the whole day they sell perishable goods (fresh fruit & vegetables, bread & bakery, milk & cheese) at reduced prices because they are on the expiry date (always check it when you buy). Sometimes they even allocate a special time and sell them at even lower prices. You can still eat most of these items and enjoy your flowers the next day, but they cannot sell them and will have to throw them in the bin. Find out when this time is and go shopping.
Actually, there are several times during the day when products are sold at reduced price, like midday, but usually the time when you can find the biggest discounts is about one hour before closing time (e.g. 7 pm if they close at 8 or later). Sometimes they change this time, I guess it's to avoid crowds (it does happen!)
They usually put a special, very visible, coloured label with the discounted price, so once you start hunting for them your eyes will spot them immediately.
However, if you don’t want or can’t spend time chasing bargains at your supermarket, you can use online ones and here too you can save a lot, have a look.