If everything goes according to their schedule it should be back online tomorrow at 10:00 GMT (so after the regular "All Worlds" maintenance).
[Date & Time]
Apr. 1, 2019 10:00 to Apr. 2, 2019 10:00 (GMT)
Apr. 1, 2019 11:00 to Apr. 2, 2019 11:00 (BST)
* Completion time is subject to change.
Lodestone link: https://eu.finalfantasyxiv.com/lodes...54031233a75d76