This is earned for participating in 31 daily challenges in a row.EDIT: This has been patched to require just 31 daily challenges total, they don't have to be consecutive. Though getting a run done every day will get this completed the soonest.
On the main menu, where you see New Game, Challenges, etc., press Tab to go to the daily challenge page. A new run appears every day at 4am Central Time.
You only have to participate each day, you don't have to win. This means that if you're having a busy day and can't play much, you can just start up the daily challenge, suicide in the first room, and you're set for the day. As long as you get placed on the leaderboard for the day, it counts.
Though if you do feel like trying, if you WIN 5 days in a row, you get
There is no counter in game, so you'll just have to keep playing until it pops.