You can't blame other customers not knowing when Valheim goes on sale. They can only assume because both the developers and the publisher are not allowed to disclose sales prior to them happening (because of an NDA with Valve). For the future, I recommend using SteamDB and checking the game's price history. This will give you an idea on when the respective game might be on sale again.

Additionally, I recommend reading reviews and watching some Youtube videos. Plus, you again have the option to buy and test the game. You can refund it if you haven't played longer than two hours and submitted the refund request < 14 days after purchase.