Welcome, AutumnWind!
The talk between Lewis, Tolkien, and Hugo Dyson you're thinking of happened on the night of September 19th, 1931. The three men went after dinner for a walk in Addison's Walk, a park alongside the river in Oxford near Lewis' college. While walking, the conversation centered on myth, the studying of which was an important and beloved part of all three men's interests. Lewis took it as granted that myth was not true, which Tolkien disagreed with. Tolkien offered that myth, like all else, came from God and that in some way presented God's Truth. Myth might distort or disguise God's Truth, but it is still there. Tolkien went on to say that the telling of myth may be a way sharing God's Truth. Tolkien left to go home at 3am, while the other two kept walking and talking until nearly dawn. Dyson added to Tolkien's words that Christianity was able to bring change to the believer.
A little over a week later, on September 28th, Lewis was riding in the sidecar of his brother Warnie's motorcycle to visit the Whipsnade Zoo. By the time they arrived at the zoo, he was a Christian.
I've shortened the story quite a bit. For the full story, you should read Lewis' memoirs, Surprised By Joy.
