I think I’ve read about this on sage-members (it is also quite possible that I’ve blogged about it, but a quick search did not reveal anything):
You’ve got a major upgrade ahead of you, one that might take too long to complete and on top of that, the company (your employer) cannot halt while you are at the task. So do you schedule to start the upgrade on Friday evening, or on Sunday morning?
For years I used to opt for Friday evening. But it seems that I was lucky. For as I read in sage-members, what if the upgrade does not complete and you need support? Do you have (verified) support 24×7 for everything involved in the process, including hardware, software, personnel (if shifts are needed)? Even if you do, have you ever tested them? Are the support people you contact on weekends of the quality you expect or simply note takers so that you get an open ticket and a checklist while an actual solution may arrive on Monday evening? What if a simple fan fails and you need to replace it?
Start on Sunday mornings. As a bonus you get a full day for rest and mental preparation.
Managing organized complexity 
Support, as well as roll-back plan must be in the check list before any major upgrade.
When I was an admin (at least 12 years ago, that is) I always preferred Saturday morning. Saturdays have two benefits: 1) You are not tired as on Friday evenings 2) You have plenty of time to deal with problems until Monday morning (that means you can use Sunday if needed).
From what I remember from those days, we were never less tired on Saturday mornings compared to the previous Friday evening :)
The point is not having plenty of time until Monday morning, but not having plenty of *idle* time (while waiting Monday morning to order a spare part for example).
No matter how big or odd a deployment can be, I vote for Wednesday. In fact, I’m quite surprised to see that no one advocates Weds; and that’s quite weird since it has a series of advantages against weekends (shops are off, people rarely cross city borders, standby engineers tend to be more active etc). In case CAT-UAT phases commenced successfully and the deployment KIT along with the rollback plan are accurate to the bone your chance to fail is too small to worry about. Hardware failures are something we deal with every single day and IMHO this can’t be considered part of the upgrade. On the other hand if the HW upgrade is the upgrade itself then it’s wise to have the system on for a while before the switch, so as to be able to promptly handle system failures or application inconveniences and be prepared for the final launch.