I wonder if that’s why it’s so difficult to look up specific stories. I vividly recall reading one in my childhood about a KotRT who sought out to challenge another who was supposed to be unbeatable during daylight, but insisted on waiting to challenge him at his best rather than his weakest. Always thought it was a great honor story and I can’t for the life of me find it.
Note that this "strength was greatest at noon" was also said to be true about Gawaine, and there is a version of a combat where after the noon hour his strength waxed and he was defeated.
"According to the Vulgate Mort Artu, Gawain had been baptised as an infant by a miracle-working holy man, also named Gawain, who named the boy after himself, and the following day announced that every day at noon, at the hour of the baptism, his power and strength will increase."
I think that's probably based on the story line about Lancelot wounding Gawain with Balin's sword. Somewhere in Morte d'Arthur, but I don't recall the "wait to be at his best" part.
It sounds like a Hollywood addition to the story, so maybe a movie?
I would suggest asking on reddit if you can't find it here. I've actually managed to find some things that I had been seeking for over a decade through places like reddit.
https://scifi.stackexchange.com/ might also be a good place to ask, story identification threads are common and often answered quickly if sufficient detail is given (or the asker is lucky).
The details I remember, the one he wanted to challenge was the Red Knight (I think) and the other detail of the story that I remember was whoever was going to challenge him initially didn't have any armor and kept collecting pieces from other knights that he defeated along the way.
I think it might have been Gawain now that you mention it. Pretty sure it started with a G so I kept thinking Gallahad.
