Considering that Jomtien Immgration only allows everybody to extend their tourist visa or visa exempt only one (1) business day prior to stamp expiry, that means anyone with a stamp expiring on the 12th, 13th, 14th, 15th, and 16th will be going in to extend on the 11th and won't get their passports back until the 17th, as it's a 2 day process. That's a very long time to be without your passport and hoping that they don't lose it in the meantime.
I'd be guessing there will be an absolutely insanely massive queue on the 11th next month. Hope they will put on extra staff to work the days in the lead-up to the extra long holidays.