I have visited Mathematics Meta 10 days on the trot, but I don't seem to have a hat for this.
Why is that?
You don't earn hats on Meta, and you usually also don't get hats for things you do on Meta, unless the hat specifically says "do X on meta". Quoting myself from Why didn't (or did) I or some other user get a hat although the requirements were (or weren't) fulfilled?,
Most hats are awarded for things you do on the main site, but there are a few* that are awarded for performing an action on the site's meta. Their requirement generally includes the words "on meta". You still earn these hats on the main site, but you earn them for doing something on its child meta.
* This year there aren't any.