You can submit your 90 Day Report up to 7 days late without penalty, so submitting it on Monday instead of Saturday will not result in a fine. It is advisable to file the report anytime between 15 days before and 7 days after the due date to avoid any fines.
90 DAY REPORTING RESOURCES / SERVICES
Use the trusted Thailand 90 Day Reporting Service to get your in-person report done and mailed to you for as low as 375 THB (even if the online system doesn't work for you).
For immediate assistance, contact Thai Visa Centre directly via LINE at @ThaiVisaCentre or Email them.
A person can file a 90 day report between 15 days before the report date to 7 days after the report date in person. If a person does not file the report, then they may pay a fine of up to 2K THB for failure to file.