There are several, one is that you can time it with payroll, so your last day is the end of the current payroll period, just makes it neat for accounting purposes.

Another is that if the company doesn't want the media to pick up the "bad news", Friday is the best time to do it because it'll get lost over the weekend when it's reported on.

And finally, there's less worry about retaliation from disgruntled employees if they have the weekend to calm down over being let go.