Ok. Further to my post about December immigration office closures it appears Friday 29th they will be closed too.
So holidays for December are:-
December 5th
December 11th
December 29th
Also Monday 1st Jan 2024 is a holiday too. That means that Tuesday 2nd January will be manic in many offices around the country. Avoid going if you can help it.
If your entry stamp or extension expires on a day where the immigration office is closed, and you go the next day that they are open you can still get an extension and you won’t be charged 500 baht per day overstay, but you will get a stamp in your passport saying something like “overstay xx number of days - fine waived due to closure”.
Land borders are open as usual during the holiday period.
Plan accordingly.
TLDR : Answer Summary
In December, the following dates are when immigration offices in Thailand will be closed: December 5th, December 11th, and December 29th, with an additional closure on January 1st, 2024. If your entry stamp or visa extension expires on a closed day, you can still receive an extension the next day without a fine, though a stamp indicating an overstay will be placed in your passport, stating 'fine waived due to closure'. It's advisable to avoid visiting immigration offices on January 2nd, as they are likely to be crowded.