Can my parents extend their 30-day visa exemption in Thailand and when should they apply?

>is that possible during this time?

The next day the immigration office is closed is Friday December 31st and Monday January 3rd 2022.

>If it is, how many days before their visa expires would they need to go to an immigration office to apply?

Chiang Mai, Bangkok, Buriram, Chiang Rai and a couple of others all allow people to apply for extensions when they have 45 days or less left on their current stamp. The other immigration offices are 30 days. However, ask your local immigration office for specific details.

And to answer the question of "what do they need to bring"?

1. Their passport and:

a. a copy of the data ("picture") page

b. a copy of the most recent visa (if you have one)

c. a copy of the most recent stamp

d. a copy of the departure card

2. [Might not be asked at Bangkok immigration] A TM30, filed in the immigration office.

3. The forms (each immigration office may have different forms, and want to use their special forms)

4. 2x passport pictures

5. 1,900 THB.