Among the various special events Nintendo announced for the Super Mario Bros. 35th anniversary was a new Ninji Speedrun for Super Mario Maker 2. The event will be 35th anniversary themed, and is coming to the game sometime in November 2020. That said, it seems Nintendo had plans to run this event much earlier.
People have gone snooping around the official Mario hub site that Nintendo put together, and they stumbled upon an info page for the Ninji Speedrun. The difference is, this page seems to be an old, unused page that Nintendo didn't think the public would find. The page lists the same Ninji Speedrun mentioned above, but it shows a release date of April 14th to 21st, 2020.
There's no doubt Nintendo hand planned to reveal a lot of their Mario 35th anniversary plans earlier in the year, but the pandemic through a monkey wrench into those plans. This defunct announcement page could show just how far Nintendo had to push some plans back.
Thanks to Wipeout Jack for the heads up!