This is most common for weekend trip where you may want to book a package and fly out on a Friday and come back on a Sunday. Make sure you always check to see what the cost is to fly back on Monday. I see this all the time for Vegas and was reminded this week while doing some research for someone.
In this case you could actually stay Sunday night and fly back Monday for less. This is because the biggest factor in package deals is the airfare and flying back on a Monday will almost always be cheaper than Sunday. For the Vegas research I was doing it was about $150 less(for two people) to come back on a Monday, so a huge savings.