Why is my house full of butterflies all of a sudden?

I came home from work two days ago to find a butterfly trying to escape through a window that doesn't open. I thought it was quite cute at first. I left the windows open yesterday so i could find its way out but came home to find it still trying (unsuccessfully) to exit through the solid glass.

I also saw that it had gained a friend, which remained still somewhat adorable. Two idiots in arms. The windows remained open

Today i have come home to them still (as I am writing this, one of them just beelined out the open window, as if it hadn't been able to do that at any moment in past 72 hours) sitting on the window ledge. I thought at first, oh dear, one of them lost a wing. Then they both flapped their wings. Oh my, I thought. There's a third idiot. Then the third idiot flew out from under the blind. With both wings attached. So to count, in the past seventy two hours i have acquired THREE able bodied butterflies and one disabled butterfly.

WHERE ARE THEY COMING FROM??