With global access of most of our systems and engineers doing work at odd hours of the day & night, all around the world, we don't have a good time for down time. So, with input from the business, it was decided that hot fixes & minor updates would go out on Wednesday's in the evening EST. Major work is done over the weekend. I'm getting ready to retire an Oracle database (and yes, I like saying that) and start the app on SQL Server 2008 tomorrow (Saturday) morning.

I used to work for a dot com that was also 24/7. After analyzing access to the system, we tried to do major maintenance at 3AM on Sunday mornings because that was the time that we had the least number of people on the system.

I can't imagine taking down a system in the middle of the work day, on purpose, unless it was a dire emergency.