What is a day?

What is a day for determining number of days a furnished holiday let has beeh rented out? 

When the property is rented out for the weekend on some occasions they tenants arrive on Friday night, stay over Saturday and Sunday, and generally leave on Sunday evening.  Does that count as 2 days of 3 days of letting?  Also, sometines they don't leave untill Monday morning, normally when there is no one coming in for the following week, so would that be 2, 3 or 4 days? 

Is it the same as for the residence tests, so you count midnights, 3 days, or is it if it is let at any  time on a day, so would be 4 days, or perhaps count hours, so if it is more than 48 hours you would count 3 days, even if the occupation spreads over 4 different days.