Can I extend my tourist visa the day after it expires without a fine if the immigration office was closed?

you are in luck. (y)

If the immigration office is closed (weekend or holiday) on the date your current stamp expires AND you go the very next day they're open you don't pay overstay and you can get your extension like normal.

Just so you know there is no reason to wait until your current stamp almost expires because they ADD the 30 day extension to your existing expiration date.