If you've got a deadline on Friday, and you email about whether it would be okay to take until Monday, and they don't answer the email right away, that means they aren't sitting around waiting for it today, and it's the same as an extension until Monday. Right?
Yesterday, I had a deadline that was "the close of business Thursday." I sent the draft at 5:59 p.m. Did I meet the deadline? It's more business-y to work late, isn't it? It's a slackerish businessperson who leaves at 5. And don't you love the 59? It makes it look as though I was all briskly focused on 6 as "the close of business." And what's with "the close of business" as a deadline anyway? If you're clocking out at the end of the work day, why not say "before the open of business Friday"? Why deprive me of the evening if I might happen to have writing habits of that sort?