I understand that the book is not 100% accurate

Well, indeed. I can't say I've read the whole book, but judging by the excerpts I've seen, I would be surprised if it is as much as 5% accurate. It is a fair approach to assume that everything in Gates of Fire is made up by Pressfield, and that any similarities to historical events or characters are more or less accidental.

The notion of a raid on Xerxes' camp is probably derived from the widely discredited alternative account of the battle of Thermopylai found in Diodoros and Plutarch. It does not feature in the more detailed account of Herodotos, or in most scholarly reconstructions of the battle, for the very good reason that it is physically impossible and clearly never happened. Xerxes' camp was miles away from the Greek position, beyond a guarded pass even more narrow than the one the Greeks were defending.

That said, Pressfield distorted even such feeble evidence as we have for the night raid on the Persian camp. Diodoros (11.9.2) tells us that all 500 remaining Greeks went along on the raid, knowing that they were surrounded and that they were going to die. There is no evidence for any small group going out on a raid, and no evidence that Dienekes was an officer or that he had any part in the night attack.

None of the surviving accounts of Thermopylai mention anything about rain. However, Herodotos tells us in his account of the simultaneous naval battle at Artemision that on the first night there was thunder and torrential rain (Hdt. 8.12). Since this battle took place on the same three days as Thermopylai, in the strait just next to the pass, it is fair to assume that there was a downpour over the battlefield of Thermopylai too, which Herodotos didn't care to mention in his description of the engagement on land. If this is what Pressfield is working with, he imagines a raid on Xerxes' tent on the first night of the battle - not the second night, where Diodoros places it. The whole scene is clearly a product of Pressfield's imagination.

As you say, the battle probably took place in August, or early September at the latest. Herodotos himself, in the same sentence where he tells us about the rainstorm, notes that this extreme weather happened because it was midsummer. It seems very unlikely that a rainstorm in Central Greece in this season would make for an "icy torrent". If we give Pressfield the benefit of the doubt, he may have meant simply that the rain was cold?

Also side question how does one pronounce Dienekes? Die-neck-es?

The name Διηνέκης has four syllables: Dee-EN-eck-es.