You can take days immediately before or after rostered days off. The only rule is that you are not allowed to leave a single day working. So if Monday to Thursday was rostered off, you could book the Friday off but you would not be allowed to book the Sat off as that would leave a single days working on the Friday. You could of course book the Friday and the Saturday off. Does that make sense?
Also if you book leave during your reserved month you will automatically get a day off either side of the days that you have booked leave on.