Is it possible to do a 90 day report anywhere in BKK on a Saturday? Chaeng Wattana or any of the BOI places?

No. But there is a +/- 7 day reporting window in Bkk from the due reporting date if doing so in person or someone reports for you.

Does that mean if less than 7 days no fine?

In Bkk you should be ok. The law does allow a +/- 7 day time window for reporting (except by mail), but there have been reports indicating some other immigration offices are doing things their own way and wanting reporting done by due date. So may be different at other offices.

I guess for sure it will never harm if you report a few days early.

Always a good plan.

I was informed by the human resources department at my school that yes, there is an option for Saturday 90-day reporting:

The old Immigration facility on Soi Suan Phlu. Saturday mornings only.

While I've shared this info with some friends, I haven't checked it out myself, nor have I heard any reports of success, so this might not be gold standard info. Might be worth looking into, though. Good luck.

I have heard that in Bangkok they will not accept anytime after your due date. It s not +/- 7 days. It is now only 7 days BEFORE your due date. 1 day after, and you have to pay the B2000 fine.

I was due to report on the 4th (January), my HR dept didn't realize and they didn't collect my passport until the 6th, reporting (BKK) was done no probs/no fines.

My 90 days report was due on 12 December(Saterday).

I went on 15 December.No problem what so ever.

(Korat immigration in Chock Chai)

Thanks everyone for the info... went in today and no problems and no payment needed, also only took 15 minutes.

Report date was the 5th and went in on the 11th