I was too ill to go today, so I'm overstayed by a day

Immigration is not opened on Saturday or Sunday in Chiang Mai. One day over gthey may be nice. Do you have doctor letter? Go Monday.

Overstay is 500 Baht a day

Did you get a doctors certificate? Yes, should be O.K.. No, Pay the fine. NO Immigration office is open Sat. Sun. or Public holidays. ALL border crossings are open 365 days a year.

rrose- for a visa extension, or a simple 90 day check in?

For the check in being over a day or 2 is a non-issue.

For an extension, yeah technically you'll need a doctor letter to avoid the fine.

But I am certain that if you are nice (as I know you are) in explaining, they'll probably let you slide.

Let us know later how things turned out, and I hope you feel better soon.

rrose if you are referring to an extension of stay you really should apply about a week early and avoid these problems.

I made a mistake in calculating how many days I could stay and tole the immigration officer this when I approached. Paid nothing. I would imagine that if you told them you were ill, it should be the same.