@Sandi ******
there is no such thing as a 30 day tourist visa :/ , if you showed up here and were stamped in for 30 days you came in on a 30 day visa exempt entry.
Be that as it may, you can go to the immigration office and apply for a 30 day extension of stay for 1900baht on Tuesday the 17th.
The rule is IF your "admitted until" stamp expires on a day the immigration office is closed (weekends or holidays) AND you go the very next day they're open you are not charged for overstay and can get your extension like normal.
You will get an "overstay during holiday no fine" stamp and your extension will be dated from the date your current stamp expires