Christmas would be too late to make changes such as adding more schools. The majority of MSTP deadlines for primary application are about October 15 or November 1. If you do not get at least 3-4 invitations to interview by October 1, you should consider adding more schools, and contacting every school who has not rejected you with an update/interest email. To be comfortable that things will work, you need to aim for 8-12 interviews. That should give you 2-6 acceptances.