At home

I've decided to post some English tips here since I've been teaching ESL a lot more, and I encounter similar issues in pretty much every class.

Here's a common phrase I hear:

"On Saturday, I stayed in my house."

This means that you are literally inside the house, instead of outside the house; you're saying that you are in an enclosed physical space. 

Instead, you have to use "at" instead of "in" because it's an activity at a location. 

"On Saturday, I stayed at home."

This person is inside a house (as opposed to outside the house).