Would it be okay for me to donate blood in September if my surgery is in December? (1st time donating) What about allergy shots?

I would definitely check with your PS. It depends on what your full blood count is and your Hb level and the amount you intend on donating. You will need a blood check preop and my concern would be that this may be low and the worst case being that you donate blood in Sept and when you have your blood count done in Dec it is low which would increase the risk of you needing a blood transfusion postop. Best to check with your PS and maybe get a blood count done now to see what your Hb level is. then you can make a more informed decision as to the pros and cons.

I would discuss this question with your physician.  Some labs do not have the capacity to store blood that long prior to surgery. If you have had bariatric surgery, it is important to know what your blood counts and protein levels are now. You might ask your family physician to check your labs now to see if you need and iron or extra protein prior to surgery.