Hello @Georgina36,
If you're happy that guests can check in on a Sunday and check out on any other day then the suggestion from @Marzena4 will work.
There's no way to ensure that guests can book such that they will check in and check out on a Sunday.
If you specify that they must check in on a Sunday and set the minimun stay to 7 nights a guest can still make a 8,9 10 etc. booking checking in on a Sunday and checking out over a week later on a Monday, Tuesday or Wednesday etc.
If you also specify a maximum stay of 7 nights then guests will have to check in on a Sunday and out on a Sunday but they can, of course, book only a 1 week stay.
Steve.