When should one apply for a Tourist Visa extension at the Bangkok Immigration office: is one week before expiration of the visa okay? Or can one do it on the same day as the visa expires? (The day before is a public holiday.)

You lose nothing by doing it early and one week should be fine. Last day is taking a chance (you may be sick).

You lose nothing by doing it early and one week should be fine. Last day is taking a chance (you may be sick).

Cheers for the quick reply, lopburi3.

How early I can apply extension?

When I enter Thailand my passport have to be valid for 6 months. How about extension? If I apply extension and my passport is valid only 4 months and week, do I get extension? Or can I apply extension next or even same day I arrive Thailand?

Immigration does not want to see extensions with less than 30 days remaining on permitted to stay stamp unless there are special circumstances involved. You can not obtain an extension for longer than your passport validity; but can up to expiration date.