Can I apply for a visa extension on the next working day if my visa expires on a weekend?

Worth to check the appointment availability on the 18th.

And if you are applying for a Covid extension, it is not a promised thing to be approved. Be ready with Plan B...

Ok, there is no such thing as a covid visa 😮 so you are on a 60 day covid extension that runs out the 17th.

You are correct, IF you have a stamp that expires on a day the immigration office is closed (weekends or holidays) and you go the very next day they are open to apply for an extension you will not be charged for overstay but you will get the overstay stamp in your passport (it will say overstay xxx days, fine waived due to closure)

Be ready for Monday to be a madhouse at all the immigration offices seeing as when they close today (in another 2 hours) they aren't gonna open back up until Monday at 8:30am (13, 14, 15 are holidays this week).

I'd say show up bright and early Monday morning. 🙂