You should discuss your submission plans with your supervisor, though ultimately it is your decision when to submit your thesis for examination. About two months before you expect to submit your thesis, you should complete the Intention to Submit Form. Completing this allows the department to make the necessary arrangements for your examination. You do not need to be overly exact in calculating your submission date, provided that you adhere to your deadline for submission.
Should you have a request for an extension to submission pending at the time of your deadline, you are strongly advised to submit a digital contingency copy of your thesis in order to avoid automatic failure for non-submission should your extension not be approved. PGR Special Cases can provide details on how to submit a contingency copy. You can get in touch via pgr-special-cases@york.ac.uk.
Theses are required to be prepared and submitted for examination as specified in the University's requirements.
You must submit your thesis by the final deadline, though it is a good idea to plan to submit long before that, in order to give yourself time to deal with unexpected events in both your research and your personal life.