Maintenance for the week of April 6:
• [COMPLETE] ESO Store and Account System for maintenance – April 8, 9:00AM EDT (13:00 UTC) - 6:00PM EDT (22:00 UTC)
This event have never been skipped and is tied to IRL event. That's as much confirmation as we have for now.
But we are promised an "outline of 2020’s remaining in-game events"on 19th: https://www.elderscrollsonline.com/en-us/news/post/58759
It was already tested on PTS with the new sugar skull weapon pack, so we can expect that it will be happening at around the same time this year. Since events usually start on Thursdays, I'd guess it will start on the 22nd of October.