I use google shopping. For Black Friday, you can indicate the promo on this ad format. But it only works if the promotion is made AT THE TIME OF THE SHOPPING.
When I put for example -20% on all my products, google considers that there is no promotion.
But if I do as google asks, the customer does NOT see the price crossed out with the promotion on the product sheets...
So, how to properly apply the promotions with google shopping?
The promotion needs to be redeemed in the checkout, if the promotion is applied before the checkout, such as the product landing page, then this is called a sale price, not a promotion.
Using this method you can enjoy automated promotions. Meaning the dropped price will show x% price drop, or it will show the old and new price.
To learn more about promotions: https://support.google.com/merchants/answer/4588460