pretty good strategy if you ask me. amazon has to have engineers working seven days a week anyways, if they push new stuff on a friday afternoon then it gets a couple days of low usage before all their customers get back to work on monday and try to implement it.
Sure, critical systems have round-the-clock coverage, but pushing big changes before the weekend is still not optimal. If there is an emergency, you'd rather have most of your workforce available, awake, and at work.