Like the title states, this is an API for NASA's Astronomy Picture of the Day.
You can use it sending a GET
request to /api/<year>/<month>/<day>
.
It also can generate a PDF, which you can GET
at /api/pdf/<year>/<month>/<day>
.
You could also just use the Web Interface, which has buttons. (yes, very fancy; I know)
See it in action here.