Can I extend my visa a day after it expires if the immigration office is closed on holidays?

Yes

Yes….

I am doubtful you're here on any visa (that you bought from a thai consulate before you winged your way here) and it's far more likely you just showed up and got stamped in free for 60 days on the visa exempt entry program. 🙂

Be that as it may

If your stamp runs out on a day the immigration office is closed (weekends or holidays) and you go the very next day the office is open (in this case Monday Dec 8th) you can get your extension like normal for the 1900baht, will not be charged for overstay but you will get an overstay stamp in your passport that says "overstay xxx days fine waived due to closure"

Just a tip, there's no need to run your stamp down to the end to get an extension, most offices (other than Jomtien/Samui) allow you to apply for an extension when you have 30 days or less left on your current stamp and you don't lose any days applying early as the extension is added to your current expiration date