This will likely depend on your market, minimum night stay, and housekeeper's availability. Below are some common scenarios of when hosts might use changeover dates. 

Scenario 1

In your market it's customary for travelers to book week-long stays. Let's say your minimum night stay is 7 days, and you don't specify a "check-in changeover day." Guests may book: Wednesday to Wednesday, Thursday to Thursday, Sunday to Sunday, and so on.

Now you have 1, 2, 3, 4, 5, and 6 day calendar gaps. 

You can sell off the gap dates–or to avoid them altogether, select a check-in/check-out changeover day. The most common changeover days are Friday, Saturday, Sunday, and Monday. Guests depart in the morning, with new ones checking in that afternoon. 

Scenario 2

Let's say you want two sets of week-long reservations over the Christmas/New Years holidays: One group will stay for a min. of 7 nights over Christmas, and another for 7 nights over New Year's.

Sometimes travelers will book 9 nights: checking in on Christmas Eve, and checking out on January 2nd.

To prevent this, you can create a calendar season called "Christmas;" then adjust the check-in/check-out changeover days and number of minimum/maximum night stays. 

Scenario 3

During the off-season, only weekends usually book and your min. night stay is 2 nights. Friday and Saturday are priced much higher than the rest of the week, and you'd like to keep those two days together.

To prevent a traveler from taking only Thursday/Friday OR only Saturday/Sunday, do not allow check-in/check-out on Saturdays.