The two Brits met up in February, 1963 and Idle admits his "life changed" on the fateful day.
He recalls performing Cleese's material as a 19-year-old freshman at Cambridge University when 23-year-old Cleese, "a senior member of The Footlights", walked in on a performance of a college revue.
Idle writes, "Imagine, then, a not particularly large room, an ex-19th Century Library, with gabled windows and leaded glass, packed with tables and candles, undergraduates and their dates dressed to the nines, a lot of wine and a great deal of smoke. A small raised platform in one corner was the stage... One of the sketches was an Old Testament Newsreader played by Bill, called Bbc Bc... I played the Biblical Weather Forecaster.
"I didn't know it at the time but that part was written by John Cleese for himself and afterwards in the euphoria a very tall man in a thick tweed suit with dark hair and piercing dark eyes was introduced to me... He was very kind and complimentary, and indeed encouraging."
Cleese urged Idle to audition for The Footlights revue club and the funnyman was voted in a month later. He recalls learning comic timing and confidence from his new pal, and performing with Cleese in London and Edinburgh - while also meeting the other leading members of the Monty Python troupe, Terry Jones and Michael Palin.
Signing off, Idle writes, "And now we're still here, and it's amazing to look back and think about it all. So thank you John, for all the laughs, all the funny material, all the support and great lessons in comedy which you generously gave to all of us who had the pleasure of performing with you. You were always the funniest, and always the most serious. I am eternally grateful for the trip. Happy Anniversary."
Cleese directed fans to the essay via Twitter.com on Wednesday (06Mar13), writing, "Many thanks to Eric for his lovely blog commemorating 50 years of friendship."