When does the two-month fund seasoning period for a Thai visa extension begin?

The rules are clear you need to transfer in the account 40K baht a month, each month, every month, for the previous 12 months from the date you apply.

Now as far as WILL you be on overstay? That is gonna totally be up to the immigration office. The rule is IF your current stamp runs out on a day that the immigration office is closed (weekends or holidays) AND you go the very next day they're open to get your extension you will not be fined for overstay but you will get an overstay stamp in your passport that says "fine waived due to closure"

It appears from looking at the calendars that all the immigration offices are closed Monday the 26th, Tuesday the 27th and Wednesday the 28th of July. SO the soonest you can go would be Thursday the 29th of July.

I have no idea what what your "continuous extension" comment is directed toward.

Also, better to count the days (60) - not the calendar months (2). Why because February is only 28 - some have slipped up over this with a pedantic officer. Yes, I know, now I'm being pedantic. When in Rome.

The seasoning of the funds is two months before you APPLY for your extension NOT 2 months before your current extension expires.

So unless you're gonna apply on the exact day your current extension ends (which makes no sense because you don't lose any time going early) the seasoning needs to be two months from the date you're going to apply for the new extension.