How early can I request a 30-day extension for my visa exemption in Thailand, and when does the extension period start?

Hi everyone! I’ve got a quick question about the 30 day extension of my visa exemption. From what I’ve read on previous posts, I need to go to IT Square Laksi for this. How early in advance of the expiration is one allowed to request the extension? And if I do go a week or two in advance, does the extension period start the day I get the extension or the day the original visa exemption expires? I’ll be on Koh Tao the week before I need my extension and then there will be the public holidays around new year, so I’m hoping to avoid going last minute on the 3rd of January (my visa ends on the 4th). Thank you in advance for your help!