Nintendo and Blizzard are yet to share a release date for Diablo III: Eternal Collection on Switch. Until we wait for an official announcement, we might have seen the date revealed through an online advertisement.
The banner above supposedly appeared online for a short time, and then vanished completely. As you can see, it shows a release date of Dec. 21st, 2018. That would definitely fit Nintendo/Blizzard's plan to release the game this year, but it's cutting things close!
UPDATE - GameStop also has the same date listed. Thanks to @averylail116 for the heads up!