When should I tell my players that the daylight spell doesn't create sunlight?

As a player, I like to find about some mechanics during gameplay, even if it means I wasted my action. As a DM, I fear one of my players will get frustrated, if they cast the spell and it has no effect against Strahd and his sunlight sensitivity. I guess the character would know, that the spell doesn't produce sunlight even if the player doesn't. How did you handle this?

Edit: we are playing 5e

Edit 2: Yes, I know they changed it in 2024. We are not playing with these new rules.

Edit 3: Thank you all for your input. I will let them know as soon as they get the spell (only Cleric, so they don't waste learning the spell like a mage would). I will not change the spell to sunlight like in bg3 or the 2024 rule set. We all know 5e, agreed on it and I won't change that mid game. And IMO (everyone can think different about that) it adds to the horror setting, when sunlight remains extremely rare.