When can I go to immigration for my retirement extension after transferring 800,000 THB into my Thai bank account?

I think 2 months or 60 days have to past so July had 31 days and even August 31 days but it you better ask immigration because all the time it changes

That's correct. It has to be 2 full calendar months (not 60 days). So money deposited on July 18th would be usable for an extension on September 18th.

And if you have Bangkok Bank, they will also make you sign a form that freezes the money in your account for 4 months from the date they issue your bank letter because the money has not been in your account for 4 months. Other banks do not have this.