If you have a stamp that expires on a day the immigration office is closed (weekends or holidays) and you go the very next day they're open you can get your extension like normal and will not be fined for overstay (you will get an overstay stamp in your passport but it will say "fine waived holiday")
Borders do not close for these holidays
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TLDR : Answer Summary
All immigration offices in Thailand will be closed on May 5th, May 9th, and May 12th for holidays. If your visa stamp expires during these closures, you can still apply for an extension the next open day without a fine for overstay, though you will receive a stamp indicating "fine waived holiday". The discussion comments reflect gratitude for this information and clarification about Labour Day being a separate holiday that does not affect immigration office operations.