Posting in advance ensures that you are matched with the most suitable students for the role in question and maximises your chances of getting students who have worked with you before.
As your sessions tend to fall within the same time frames week-by-week, you will be able to build up regularity with your favourite students as their timetables generally coincide with the times you need them. Posting in advance ensures that these students are able to prioritise and accept your Stints first.