You have to take note if there is a black border around the green 3 day window of dates. That is the only way you can select a shop.
Only the day you are in and the next 2 days are allowable for most jobs - ie: the 3 day window, even if there are green boxed dates if they don't have a black border you are not eligible for them for some reason.
It is kind of complicated as they have all these color codes that are meaningless to us if we can't select them anyway.
Sometimes you can contact a scheduler and get them assigned manually, it just depends on the job / deadline / reshop period etc.