National Holiday 18th July?

I'm trying to book a hotel in Kyoto on 17-18th July.
The 18th is a National Holiday.
Is everyone free and will it be REALLY busy?
I wanted to go to Osaka Aquarium on that day. :-(

I'm trying to book a hotel in Kyoto on 17-18th July. The 18th is a National Holiday.

Not only is it a national holiday, the 17th is also Gion Matsuri.

Is everyone free and will it be REALLY busy?

You can expect the popular spots to be quite busy, including the Osaka Aquarium. There will probably be waiting time to enter the aquarium.

The national holiday also happens to be Ocean Day. Maybe that makes the aquarium particularly busy.

Uji,
In your opinion, is it TOO crowded that weekend? Right now, I am planning on starting in Tokyo on July 9th, and being in Kyoto from the 16th to the morning of the 20th. I thought the Gion Matsuri would be a fun to see. But we won't to Kyoto until sometime in the afternoon on the 16th, so we won't see much that day, and the matsuri will take up most of the 17th. So we really only had one full day to do our sightseeing in Kyoto, because we also wanted to go to Nara on the 18th or 19th.
Would we be better off skipping the matsuri and going to Kyoto a few days later? And if we do go to Kyoto that weekend, which would be better for Monday the 18th, Marine Day -- sightseeing in Kyoto (probably Kinkaku-ji, Ryoanji, and Arashiyama) or going to Nara? Or will both be equally crowded? Thanks for the help.

In your opinion, is it TOO crowded that weekend?

In my opinion, it is never too crowded in Kyoto, especially if you avoid using buses.

But we won't to Kyoto until sometime in the afternoon on the 16th, so we won't see much that day, and the matsuri will take up most of the 17th.

The highlights of the festival are in the evening of the 16th and the morning of the 17th. The events on the 17th don't have to keep you busy for an entire day. You can do some regular sightseeing, as well.

And if we do go to Kyoto that weekend, which would be better for Monday the 18th, Marine Day -- sightseeing in Kyoto (probably Kinkaku-ji, Ryoanji, and Arashiyama) or going to Nara? Or will both be equally crowded?

You will rarely encounter any of these places free of crowds, unless maybe you visit early in the morning or late in the afternoon. It can be enjoyable, nevertheless (except Ryoanji, which is difficult to appreciate with big crowds).