It's not really a bullpen day, they're just trying to reduce the number of times the starter having to face the top of the order. The starter comes in in the 2nd inning. I doubt they'll do it with all their pitchers, but it is kind of interesting.
I'm not confident enough in the data yet (plus it continually changes as lineups are posted, etc). That's why I made a spreadsheet people could use make their own.