@Jarek ********
If you don't meet the requirements and don't get the extension, and you do it on the last day, you'll get a "7-day to leave" stamp. But it's a tourist visa extension. The likelihood of being denied are almost not possible. Going earlier or later will have no effect on the chance of getting a visa or not.
Not going on the last day is simply better for you, because if you're missing a document, or your transportation breaks down, etc., you won't end up on overstay, which isn't good at all. Better to go a week or so before.