If it's neither day nor night, it becomes day as Obsessive Astronomer enters the battlefield.
Whenever day becomes night or night becomes day, discard up to two cards, then draw that many cards.
The information presented on this site about Magic: The Gathering, both literal and graphical, is copyrighted by Wizards of the Coast.
This website is not produced, endorsed, supported, or affiliated with Wizards of the Coast.