Why are classes not held on exam days?

I've read that regular classes aren't held on exam days as well, mid-terms exams and final exams. Is is that instead of having regular classes, students normally are doing the test at that time? and if so, how come it takes 2 days to finish the mid-terms test and three days for final exams?

Is is that instead of having regular classes, students normally are doing the test at that time?

Yes.

and if so, how come it takes 2 days to finish the mid-terms test and three days for final exams?

I don't know where you got those numbers from, but the number of days are not always 2 and 3. Before I answer any further, may I ask why you have been asking all these questions about exams? And could you tell me how old you are and what country you go/went to school at? I feel the need to know these answers in order to explain the situation effectively to you.

Alright I'll explain it as effectively as I can. I started being home-schooled by my mother when I was little and now I've reached 10th grade--meaning I'm 16. Because of this, I never attended high school so I'm filled with curiosity about it. But then, why am I trying to find information of Japan high schools? I'm just planning to write a story of a Japanese high school EVEN though I found out that many people are actually doing these. I forgot to add, but I'm home schooled in the United States not in Japan.
So you might be reading a lot of questions I ask just for the sake of my story, but at the same time--out of curiosity!

Thanks for your feedback. That makes sense.

First of all, let us think why final exams are necessary in the first place, or why terms exist.

Children go to school to get educated, teachers are supposed to educate them, and parents are supposed to support them, so that the children can finally contribute to the society when they become adults.

But one can go to school and just sit there the whole day daydreaming, and at the end of your 6 or 3 years of schooling, you might find yourself not being educated at all. You may even fail to graduate.

To prevent that, schools divide each year into terms (or semesters as some may call it), so that there will be more checkpoints. In Japan, some schools may have 3 semesters per year while others only have 2.

At the end of the semester, you get your grades. By "grades" I mean like "a B for Math", "an A for Reading" etc., and you are graded by how well you participate in class, how well you do your homework, and how well you do in tests.

By the way, a lot of schools indicate grades by numbers from 1 to 10 or 1 to 5 instead of ABC. And students receive a card that's about the size of a little notebook. The card will also have the teachers' comments on it.

Students look at this card, and then show it to his/her parents to get their hanko (which is like a signature). By doing so, they can find out if the way they've been doing their homework is okay, or if they need to work harder the next semester or what not. Parents make sure their children work on whatever is required.

Now, the final exams are a great way for both students and teachers to see what they've learned and to have them graded. The exams also gets you prepared for entrance exams that students need to take if they want to proceed to higher education.

The system of final exams only start from junior high school. Elementary schools typically don't have that system. Additionally, from junior high, students will start learning from different teachers depending on the subect. So there will be one teacher for Math, another teacher for Reading, then another teacher for Social Science, and so on.

Meanwhile, some teachers don't even offer final exams. For example, a music teacher might test the students' singing ability during class, but will not make paper tests for the final exam period. Even teachers of major subjects such as Reading sometimes make students write reports during class and grade that instead of making a paper test for the final exam period. And one teacher may do final exams for one term, but don't on another term.

Similarly, more teachers would be making final exam tests, while less teachers make mid-term exams. While I did mention that the exam system is effective, going through tests during a condensed perod of time is indeed hard work for both student and teacher.

Finally, the number of days used for each exam period differs greatly depending on the year or school. This is a schedule I happened to pick up by Googling.
http://www.et.hum.titech.ac.jp/~matsuda/math3/1kai/year.pdf

From the left row, it's April, May and so on, and here is a short vocabulary list.
休み day off
中間試験　mid-term exam
期末試験　final-term exam
補講期間　extra classes for those who failed to get enough
終業式　end of term ceremony

Hope it helps.

Students look at this card, and then show it to his/her parents to get their hanko (which is like a signature). By doing so, they can find out if the way they've been doing their homework is okay, or if they need to work harder the next semester or what not.

This card you're talking about, is it given to elementary school students or for high school students as well?

The card is called "tsuuchihyou" and it's an equivalent to the "report card" you have in the U.S. They are given to all students from elementary school to high school, and as far as I know, most if not all universities have them too, except that the university versions look a bit different (they're often ordinary paper instead of cards).

In the recent decades, the ones for elementary schools have been named "ayumi" meaning "progress/footsteps", but it's just the naming that's different.

Here are examples you can find from Googling images of the card. It's from mere Googling, so some are fake or joke.
https://www.google.co.jp/search?q=%E9%80%9A%E7%9F%A5%E8%A1%A8&source=l...