Add an extra available date

One of my services runs twice a month. Exceptionally, on some occasions, I have to add an available date.
I thought of doing it through the employee's availability calendar, but I find it all available by default, and then I would have to make all the dates of the year unavailable, one by one. I'm sure there is another solution, but I can't find it ....

Hi Cristina,
Actually, in default, the employees will available all dates. And OS Services Booking only has feature to setup unavailable dates for employees.
Thanks
Dam