As I mentioned in another thread, it was said by other players in the in-game chat that other servers had their maintenance scheduled for April 13th 02:00 CEST. That would be April 12th at 20:00 EDT (which is when it actually started). It is my guess that whoever wrote the notice for our homepage adjusted the times to our time zones, but failed to adjust for the day change and just copy-pasted the 13th..
Taken from https://www.thesettlersonline.com/en/homepage:
Dear Settlers,
On Monday, 13th of April, an IT infrastructure task will require the game servers to be taken offline.
Please plan your game activities accordingly.
Date: 13.04.2020
Timeframe: approx. 02:00 CEST - approx. 12:00 CEST
Game: All gameworlds offline
Adventures & Premium: Times will be extended