What does the 30-day rule say?
Since 28 May 2022, every EU country applies the same basic rule. When you announce a price reduction, you must show the prior price, and the prior price is the lowest price you applied in a period of at least 30 days before the reduction. That comes from Article 6a of the Price Indication Directive, added by the "Omnibus" Directive (EU) 2019/2161.
The European Commission's guidance lists what counts as announcing a reduction: a percentage off, a crossed-out "was" price next to a "now" price, and explicitly "Black Friday offers" that suggest a lower price. A general slogan like "best prices in town" isn't covered by this rule, but it still has to be true under the rules against misleading advertising.
How do I calculate the "-30%"?
From the 30-day low. In September 2024 the EU Court of Justice decided the Aldi Süd case: a supermarket had shown bananas at €1.29 with €1.69 crossed out and a percentage off, while the lowest price in the previous 30 days was already €1.29. The court said the percentage, and any claim that suggests a saving, must be based on the 30-day low.
A worked example for a shop selling lamps:
| Date | Price of the lamp | What you may show on Black Friday |
|---|---|---|
| 1–20 October | €80 | |
| 21 October – 10 November | €70 (autumn promotion) | |
| 11–26 November | €80 | |
| 27 November (Black Friday) | €60 | Was €70, now €60: -14% |
The autumn promotion is inside the 30 days before Black Friday, so €70 is your reference price. Showing "was €80, now €60, -25%" would be a misleading price announcement, even though €80 is the price most people paid.
Where does Shopify trip shops up?
In Shopify, the crossed-out price is the compare-at price, a field you fill in yourself for each product or variant. Shopify doesn't look at your price history to fill it in. Its own help pages on the Pricing Indication Directive say it's up to you to make sure the compare-at price is the lowest price of the last 30 days.
Three common mistakes:
- An old compare-at price stays on. A compare-at price from a summer sale is still set, so the product shows as discounted long after.
- The advised price as "was". Several countries, Belgium among them, only allow the reference price to be crossed out, not a manufacturer's advised price.
- A price raised in early November. Raising prices two weeks before Black Friday and then "discounting" them back is exactly what authorities look for.
There are apps in the Shopify App Store that track your prices and show the 30-day low next to a discount. They can help, but they only know the prices from the day you install them, so install one at least 30 days before your sale. With Black Friday on 27 November, that's 28 October.
Do authorities actually check?
Yes, and every year around Black Friday. A few examples from the last two years:
- Belgium: in its Black Friday and Cyber Monday check of 2025, the FOD Economie found problems at 16 of the 23 webshops it checked (69%). The most common one was a reference price that was too high.
- Netherlands: the ACM fined five webshops a total of €621,000 for fake discounts (published June 2024). Before Black Friday 2025, it found misleading price statements at 18 of 24 large sellers.
- EU-wide: a coordinated check of 314 online traders, published by the European Commission in March 2026, found 30% showing discounts wrongly.
Small shops are rarely the first target of a fine, but a complaint from a competitor or a customer is enough to start a case. In Germany, the Wettbewerbszentrale, a business association, takes shops to court itself.
What about the sale after Black Friday?
The 30 days keep running. If you sell the lamp at €60 on Black Friday and run a December sale, €60 is now your 30-day low. In 2025 the Wettbewerbszentrale took a shop to court for showing December discounts against prices from before its "Black Week", arguing the Black Week price was an exception. The court signalled it didn't agree with the shop.
The same goes for Cyber Monday (30 November 2026): if Black Friday's price was lower, that's your reference.
Does every country do it the same way?
The core rule is the same everywhere. The exceptions are national choices:
- New products that you've sold for less than 30 days: the Netherlands lets you state the shorter period you've sold them for; Belgium uses the previous 7 days; Germany's § 11 PAngV has no such exception.
- Progressive reductions (a price that keeps dropping during one sale): Belgium and the Netherlands let you keep the first reference price, within limits.
- Perishable goods have their own rules in some countries.
If you sell into several countries from one shop, the safest choice is the strictest version: the lowest price of the last 30 days, for every product, everywhere.
A checklist before you switch your sale on
- Export your products and clear every compare-at price you didn't set for this sale.
- For each discounted product, look up the lowest price it had since 28 October. That's your "was" price.
- Work out the percentage from that price, not from your usual price.
- Don't change regular prices in the weeks before Black Friday.
- Write down what you did and when. If anyone asks, you can show it.
- After Cyber Monday, take the sale off and remember that your Black Friday price is the new reference for December.
When this isn't the right approach
If you sell only outside the EU and EEA, these rules don't apply to you, though similar rules exist in the UK and elsewhere. And not discounting at all is often the better choice for a small shop. A gift, free delivery or a bundle protects your margin. The Commission's guidance treats conditional offers, like "buy one, get two", as outside the 30-day rule, though they still mustn't mislead. It doesn't name gifts, free delivery or bundles, but they're closest to those conditional offers. Just don't present them in a way that suggests your prices have come down. There are more ideas in Black Friday without big discounts.
A shopper who looks at the same product three times usually isn't waiting for a bigger discount. They have one question: the size, the delivery date, whether it makes a good gift. Answering it can do more than another 10% off. That's what Signal does for online shops: it notices who is hesitating, asks one short question, and shows the answer or the offer that fits. More on that on our page for online shops.
Corrected on 19 October 2026: we first wrote that a gift, free delivery or a bundle is never a price reduction. The Commission's guidance doesn't say that in so many words; the paragraph now says what it does say.
