Hyatt is an American-based company out of Chicago. The fact that the Hyatt.com website and related websites is "down for maintenance" during the end of the standard business day CST is not a good situation. Hyatt's internet-facing server and infrastructure operations groups are likely staffed heavy during normal business hours (8am-5pm) CST and rely on on-call and NOC-based support afterhours. So while the hotel reservation business is 24/7 and the dot-com operations is a 24/7 division, I will guarantee you that it is not staffed at full capacity 24/7. The "maintenance" splash page is obviously not indicative of planned maintenance.

And for the record, I am in heavily involved in an infrastructure operations division for a heavy traffic website (a lot more traffic than Hyatt's websites), so I have a decent handle on how things operate. In fact, I had a nice conversation with a colleague earlier today that used to work for IHG and it is simply amazing how little work goes into hotel companies' internet-facing infrastructure and related operational support.