We're going onhaving a national holiday this Sunday.

They want to return to their hometown, so they'll take two more days off.

I'm visiting a beautiful mountain in a village with my hiking friendpartners this Sunday.

We're going on aaway during the national holiday this Sunday.

Holiday types can be tricky!!

"National Holiday" refers to a case where the entire country is off work. For example, Golden Week is a national holiday. It does sound like you are using the right holiday type based on your post.

However, in English, we do not say "going on a national holiday". This is because we might not "go" anywhere (many people stay at home during national holidays).

Example sentences about national holiday:
"We're off work on Monday because it's a national holiday"
"Ah, the bank is closed, I forgot it's the national holiday today"
"I'm going to visit Osaka during the national holiday"

Some of my friends have already started thiseir holiday.

They want to return to their hometown, so they will take two moreadditional days off.

I think it will be good because there will be anotherdifferent view of the scenery in the rain.

We're going onto celebrate/ have a national holiday this Sunday.

It's called the Mid-Autumn Festival holiday and it will last three days.

Some of my friends have already started thiseir holiday.

They want to return to their hometown, so they will take two more days off.

I think it will be good because there will be another view of the scenery scenery will look different in the rain.