When should the 800,000 baht be in the bank for a retirement visa renewal in Thailand?

I believe in both cases, and as exampled by Tod, it is full calender months, not accumatilive days (equating to an average month), therefore March 1st.

In Chiang Mai you can apply for a new yearly extension when you have 45 days left on your current extension.

The seasoning of the banked money method is 800K baht in a thai bank account in your name only for 2 months before you APPLY for the extension (not from when the current extension runs out)

so if you're applying 3 weeks before your current extension expires or about Nov 21st you would need to have the 800k baht in a thai bank account buy the end of Aug.

The three months comes in to play because you need to keep the 800K baht in the bank account for 3 months after your extension is granted