First of all, I know this is a temporary message and would last just for a day (I hope so...), not to mention having correct information RIGHT HERE.
Anyways, the fact is on Gysahl World it can be read the following:

"From 2:00 p.m. to 5:30 p.m. on Jul. 21 (PDT) / 21:00 on Jul. 21 to 23:00 on Jul. 22 (GMT), FINAL FANTASY XIV maintenance will be performed.
During this period, FINAL FANTASY XIV will be unavailable.
See the Information section for more details."

I presume 1.18 is a big patch, but an entire day for it to be implemented seems odd for me haha.

PS: Please, don't feed any Basilisk with this poor Highlander if you think I'm nitpicking; I really don't mean to...