Even at weekends, the study rooms can be busy so it's always advisable to book the room you want in advance if you know you are going to need it. You can make your booking here. However, if you are willing to take the chance, you could always just turn up on the day.
In the Philip Robinson Library, the group study rooms and accessible study rooms are all bookable throughout term-time, including at weekends. Individual study rooms and group study room 431 (Level 4) are available on a first come, first served basis, as is YourSpace on Level 1.
The Walton Library group room is available for booking here: http://www.ncl.ac.uk/library/about/study#walton