If there are dog days, why aren't there cat days?

All days are cat days.

They have caturday. Isn't that enough?

The saying "dog days" is related to astronomy, and how a certain star appeared in the sky during the warmest months.

edit: Maybe we can consider all Fridays cat days?
It being Frigg/Freya's day, and Freya is known for her 2 kitties.

IIRC, the “dog days” are when the “Dog Star” Sirius rises before the sun does.
- Alright, don't get all sciencey 😛
There are three feline modern constellations: Leo, Leo Minor, and Lynx. They're adjacent, so they're all visible around the same time.
Leo's heliacal rising from mid-northern latitudes is around early September. Alternatively, it is most easily visible in the evening in early May. So either early May or early September could be considered the "cat days"!
That sounds like a good idea.

Why are cats and dogs considered "opposites"?

Because you have two choices when choosing a family pet, Cat or Dog. Two choices makes them opposites.
Obviously this is not all logical or rational but that's fine. People are not all logical and rational.
- Why if you have an aardvark as a pet?
Generally, thet hate each other. It's not always the case, but if you put a random cat and a random dog together in the same room, it's not going to work out well. Also...
Dogs want to be your friend. Cats might consider letting you be their friend, sometimes, when they want something.
Dogs might mourn for an owner that have passed. Cats would eat their dead owners.
Dogs may bark and warn their owners of danger. Cats would sleep through any danger.
- It's normal territorial behaviour, and afaik the signaling of their tails is the complete opposite, means they don't understand each other. Also, have you put two random cats in the same room, maybe one both of which see it as their home?
  Dogs just accept you into their pack. Ideally as the leader. Doesn't mean they will, and many dogs won't see you as superior. So they won't be friendly if you act up. Cats just accept you in their territory. Because you provide food, hopefully. They may not accept other people in their territory.
  Dogs do that too.
  Cats wouldn't sleep through, otherwise they wouldn't have survived till now, but how would you hear a soft meow in your sleep? Our cat slept in our bed, wherever she wanted, and we noticed when she got up.

Every day is Cat days if you're lazy enough.

i take catnaps during the day so

There are like 3 cat days in the year, look it up.