This information is from the State of the Network:
Thursday March 18th, 2:00 AM – 3:00 AM PST
Players may experience temporary service interruptions due to required network maintenance.
Thursday March 18th, 6:00 AM – 9:00 AM PST
The servers will be taken down to revert several PvP changes as well as fix bugs with March content. The details of the patch can be found here.
Tuesday March 23rd, 6:00 AM – 11:00 AM PST
Darktide, Harvestgain, Morningthaw, and Wintersebb will be brought down to perform scheduled database server maintenance.
Thanks for the info, Ibn!