There is no preference given to applications filed earlier than others.
Supervisors hire and select students on criteria relevant to their department.
If a supervisor is interested in hiring you, he or she will contact you directly. It will not be necessary to fill out a new application every time they are available for a particular semester. However, you may come in at those times and update your application with any needed changes. References or recommendations are not required for consideration for a position, but students may attach any supplemental information to the regular application. If you do not receive a job the semester you applied, and you are still interested, you must reapply each semester to be considered again.