It is 5:50 PM and you try your chances to take that sector (you save before trying, of course) but are not successful. So the day ends, your mates are injured or shot to death.
Moreover you find out there is no way to restore your quicksave.
Next time play it safe, idiot.
(correct?)

Yeah, you can save you day at the start, but during the actual day the quicksaves get overwritten. So if things go really bad it's probably best to restart that day.