I think I've seen at one point that if a visa extension expires on a weekend or holiday when Immigration is closed, you can apply for another extension on the first subsequent day that Immigration is open.

E.g. if the expiration date is Saturday or Sunday you can still apply for another extension on Monday.

Is that correct?

According to the dozens of posts and responses asking the same question, yes. Also, usually noted, one doesn't lose any days if they go early.

Removed an off-topic post.

Yes you can if the immigration is not open due to a holiday or a weekend, but why wait until after to do it?

Do it early, you will not lose any time as your extension , when approved, will simply be added on to your current extension anyhow.

No need to wait until the last minute,