I want to start doing business as an S Corp immediately, what election date should I choose for form 2553 (line E)?

It's the EARLIEST of the date your LLC first had owners, first had assets, or began doing business. 

I note that you state you are/were confused about the election date. I suspect (and I may be wrong with my suspicion) that you may not be fully aware of the tax implications as well as the filing requirements and other requirements (such as required minimum draw) imposed on an S-Corp.

Now my screen shows you're using the online version of TurboTax. You flat out can not use any version of the on-line TurboTax to complete and file the 1120-S Corporate tax return. So if you want to be treated like an S-Corp from day one of your business, you can't report your business income/expenses on SCH C as a part of your personal tax return.  You may want to consult with a tax professional before you do anything. This would be particularly important and necessary if your state also taxes personal income, as doing things wrong would be a double-whammy for you - and you don't need that in your first year of business.

I only created a turbo tax account to ask a question, not to handle my S-Corp filings here.