The immigration office in Thailand is closed on Saturdays and Sundays, operating only from Monday to Friday. However, if your visa stamp expires on a holiday, you can go to the immigration office on the next working day without incurring a fine, although it is advisable to visit before the expiration. Additionally, visa extensions are available at Bumrungrad hospital on Saturday mornings.
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You can get extensions at Bumrungrad on Saturday mornings. See here