Do I need to do a 90-day report for my DTV visa in Thailand, and when should I go to the government building?

Yes, you need to do the 90-day report for your DTV visa. You can report in person 15 days before or up to 7 days after your actual due date. Since your 90th day falls on a Saturday, you can report in person on Friday the 6th. Alternatively, you can also opt for postal notification within the designated timeframe to avoid a trip to immigration.