I’ve updated the pinned post re August closures.
*NOTE immigration offices will be closed from the 28th July until the 2nd of August. They’ll re open on the 3rd August. That’s 6 days of closures. The 3rd will be really busy at many offices.
If you need an extension but your stamp expires during these dates you will be able to get an extension as normal on the 3rd and you won’t be fined but you will get a stamp in your passport saying “overstay. fine waived due to closure”.
TLDR : Answer Summary
The immigration offices in Thailand will be closed from July 28th to August 2nd, reopening on August 3rd. If your visa stamp is set to expire during this closure, you can still apply for an extension on August 3rd without incurring a fine. Your passport will be stamped indicating that the overstay fine is waived due to the office closure.