I spoke to admissions today regarding the email "your application is ready for review" and interviews and here is the info she gave me. I thought I would pass it along so we all get some peace of mind
She said all applications are reviewed by a faculty member and there are over 600 applications and only about 200 people reviewing. Therefore, some people's applications are still waiting to be assigned to a faculty member to review. You will receive the email above when your application has been assigned to someone. That is why some people may have not gotten that email yet.
Some faculty members review quickly, others take their time. They then decide if the person should be invited for an interview, needs more consideration, or not for the program this year. There is no method in deciding who's applications get assigned and reviewed first. There will probably be 12 interview dates (the first being 12/15) and we should hear something about the status of our application by mid-January (interview invite, rejection, etc.) so don't worry until then if you don't hear anything, but feel free to check in if you want.
I felt a lot better after speaking to her; I'm hoping this info will help any of you that were kind of stressing. Good luck everyone!!