I'm not sure that other countries also have such days, while wthese days. We usually have one or two public holidays eachper month here.

Though I'm not sure that other countries also have such days, whilethese types of holidays, we usually have one or two public holidays each month here.

"Have such days" sounds as if you are saying "I'm not sure if other countries have holidays". Small modifications to sounds more natural.

Added "though" at the beginning to make use of the comma structure better in your sentence.

It is the third Monday in July, and it's literally it's a day for the sea.

I'm not sure thatif other countries also have such days, while we usually have one or two public holidays each month here.

I'm not sure thatif other countries also have such days, whilebut we usually have one or two public holidays eachvery month here.

I'm not sure thatif other countries also have suchthese kinds of days, whilebut we usually have one or two public holidays each month here.

Your phrasing is just slightly awkward.

I changed "that" to "if", because in my opinion, "I'm not sure that" is more used when you actually are sure, but are being humble or sarcastic.

"Also" is grammatically correct, but "too" at the end of a phrase is much more common. I feel like people use "also" when they want to give emphasis. For example, "I'd also like a coffee, if you're making one for everyone else." I didn't change it because you might want it to have that meaning.

"Such days" sounds old-fashioned to me, like a book from the 1950s (haha). I'm sure people still use it, but especially with young people, I think we just say "this/these kind/s of" instead.

I changed "while" for "but" because "I'm not sure if [it's like this for A], but [as for B...]" is a standard sentence pattern. For example, "I'm not sure if Sam is coming, but John is on his way."

For example, the third Monday in September is Respect- for- Senior- Citizens Day.

I wasn't sure if quotation marks should be used, but I followed the style of the UN on this website:
https://www.un.org/en/observances/list-days-weeks

As its name suggests, it's thea day for everyone to show respect for seniors.

It's hard to know (or explain!) when to use "the" vs. "a". I think if it's "the worst day" or "the first day", "the most amazing day", any superlative adjective, you should use "the". Otherwise, it should be correct to use "a". I can't think of any other reason to say "the day".

It i's the third Monday in July, and literally, it's a day for the sea.

You could also write "it's literally...", but I think putting "literally" first (as you did) emphasises it's true and not exaggarated. A comma should be after it, in the same way you put a comma after "basically" at the start of a sentence.

Also, it's not an error, but you'll sound more natural if you use contractions (like "it's", "what's", "isn't") every time you possibly can. So, "It's the third Monday..."

As you know, Japan is surrounded by the sea, so it's thea day thatwhere we appreciate the ocean and think about its environment.

"Surrounded by the sea" is grammatically correct, but to me "surrounded by sea" or "surrounded by ocean" feels more like what people would say.

You can also say "a day when", and that might be easier to remember since you'll already know that "when" is for time. But "a day where" is pretty common too.

I'm not sure of the meaning of "its environment". If you mean admiring its beauty and how amazing it is, you could say "its natural wonders" or "its natural beauty". If you mean thinking about how to protect the ocean and animals, you could say "its environmental wellbeing" or "its ecology" or "protecting the environment". If you mean the different biomes, plants and animals you can see in the ocean, you could say "its marine environments" or "its biodiversity".

However, for most people, such holidays are just happy resting dayspleasant days to rest.

"Such" is okay here. It sounds formal, so if you want to sound more casual, you could say "these holidays".

I think "happy day" is just used for really amazing days like a wedding. "Pleasant" sounds a bit formal but I think it's the closest to the meaning you want. You could also just use "nice". In casual speech, we tend to just use the simplest words possible, but you can definitely use more complicated words sometimes if you want. But if you use a lot of big words, you'll sound a bit formal or smart (haha).

I changed "resting" to "to rest". I'm sorry but I can't explain why. I think because you use "to do" when you say the purpose of something, eg. "This is the phone number to contact emergency services".