Yes. In the UK they go up a month before, because we have consumer protection laws that required retailers of e.g. white goods to have maintained the higher price for any announced discount for at least a month. So you knew that sellers thought there would be (at that point unannounced) Deal Days, because the prices of some things went up 20% back at the beginning of September.

I had ordered a new wireless charger about two weeks ago (without Prime). a day or two before it was meant to be shipped (about a week before Prime day), my order was cancelled "due to lack of stock". I go to the original product page and what do you know, it's in stock but $3 more expensive. Looking again today, it's on the Prime sale for the old price.

Scummy behaviour at best, and illegal at worst.

There's https://camelcamelcamel.com to check how prices developed.

There's still the possibility of shenanigans with different models that can be overpriced for long so they look like a good deal when they get a normal price