When do I share my new diagnoses with my employer?

I have worked for the same company for eight years. I have always received good evaluations, many raises, and some promotions. Over the last year, I have been in and out of work on STD with what I thought was depression. Over my last episode I went to an outpatient facility where I was diagnosed with Bipolar. I have been receiving medication and doing much better. I am scheduled to go back to work in a few weeks and want to share my diagnoses with management so that I can be covered under ADA. I am currently out with no coverage under FMLA as I have exhausted it, I am only out and being covered by STD and companies willingness to take me back. My question is do I share my diagnoses before I return to work or once I return to work?