The commenter known as Aqua Buckeye was right in submitting this Zatara story from Action Comics #17, cover-dated October 1939:
So this means that Aqua Buckeye is the winner of the variant Brightest Day #17. But thanks to the extreme generosity of F.O.A.M.er Joe Slab, both Aqua Buckeye and the original winner, F.O.A.M.er Robert Robinson, will be getting copies of that particular book! Thanks Joe, you saved the Shrine from even more embarrassment!
My apologies for the confusion and jumping the gun a bit on this particular contest. The Shrine is always striving to present the most accurate information possible, and I think my enthusiasm for this whole thing got the better of me. Thanks to everyone who participated and, again, special thanks to Joe Slab for his supreme generosity and thoughtfulness.
Update Update: Just as I was writing this, I got what looks like an even earlier example of Atlantis showing up in a DC comic, courtesy F.O.A.M.er Aaron Bias, though some part of me doubts its veracity. You be the judge:
(Special thanks to F.O.A.M.er Siskoid for the Action #17 scans!)