If you submit your Dissertation at the usual time (in mid April) you can expect the examiners to consider you at a meeting in June (check with the Mathematical Finance Administrator for details of examiners' meetings). You can then expect to graduate by July. Your Graduation Ceremony will take place at the iconic Sheldonian Theatre.
Soon after you receive your degree results, you should receive an automated email inviting you to book your graduation ceremony; if you do not receive such an email, please contact the Degree Conferrals Office, firstname.lastname@example.org.
Graduation is not only the time at which you can formally receive your degree certificate, but it can present a great opportinity to invite friends/relatives to share in your Oxford experience!