Can I submit my 90-day report on Monday if it falls on a Sunday?

You can file a 90 day report in person from 2 weeks before the due date until 1 week after the due date with no penalty.

Once you're more than 7 days late the fine for late reporting is 2000baht (it doesn't go up, so it's 2000baht whether you're 8 days late or 8 months late)

If your due date is on Sunday you can definitely go Monday without any issues 🙂