Originally Posted by Volarmis

Each week there are 2 days with maintenance - Tuesday (for main event 1st and 2nd stage) and Thursday (for reissue 1st and 2nd stage) along with their gachas.



Depends on what you mean by "maintenance". Any change done to an application could be viewed as "maintenance" and should go through quality assurance first, thus making the offline status last for a bit.

Adding to that: Clear your browser cache after these maintenances! You'll save yourself a lot of non-existing bugs (they're not bugs, they're caused by the player using an outdated and not perfectly compatible version of the client against newer data on the server).
For Firefox, go to Tools - Options - Privacy & Security tab, then hit the Clear Now button.