My non immigration O visa is expires on Jan 05 2010, so normally I go to the immigration centre in Bangkok and make my application 4 weeks prior to my expiry date. This year December 05 is a Saturday and the Kings birthday, I promised my step daughter ages ago that I'd take her up north for a holiday so maybe I won't be back in Bangkok until December 08. What's the least amount of time that I can apply for a new visa prior to the expiry date. Thanks.

January 5. But you should do a week or two early. There is no reason to do a month early (and normally they will not allow earlier than that). So no problem - have a good trip.

You appear to be talking about a 12 month extension of stay. Not a Visa.

You can wait until the last day if you wish but always a good idea to go early in case of problems.