I've just figured out how to do this myself. First, go to...
'Listings'
'Manage listing'
'Availability'
next to 'Trip length', click 'Edit'
At the bottom of that box in teal it says, 'Add A Requirement'
in the 'During' dropdown select 'Specific Dates',
As a made up example, let's say I am going on holiday for 9 days, from Saturday 5th May to Sunday 13th May, so I don't want to any guests to arrive at that time (as I can't let them in & welcome them) but am happy for them to arrive before, be there whilst i'm away and leave whilst I'm away. So, to achieve this I need to create two 'Requirements' (rules).
My first requirement is...
...I select from Friday 4th May (the day before I'm leaving) to Thursday 10th Mayand in the 'Check-in day' dropdown, I select 'Only Friday'. Note: put the 'minimum stay' as whatever you want, I decided on 4 days minimum stay, as a comprimise between my usual 2 and the ideal possible of 9, to hopefully encourage longer visiters.
My second requirement is...
...I select from Friday 11th May (the day after the previous requirement ends) to Tuesday 15th May (two days after I get back, as I don't want anyone arriving the day after I get back from holiday, and to give me 48 hours to catch up with laundry etc.) and this time I select the 'Check-in day' dropdown as 'Only Tuesday'.
So, what I end up with is that if anyone wants to stay in the 12 days containing my 9 days away, they absolutely can stay for even the entire 12 or more if they like. But they will only be able to check in on the day before I leave (or earlier) or two days after I get back.
I would also slightly increase the minimum stay amounts for the few days before that Friday too.
Hope this helps
Bella