The 100th episode of Grey's Anatomy.
Amy Gumenick who plays Becca Wells and Jeffery Dean Morgan who plays Denny Duquette both starred in Supernatural (2005) playing young Mary Winchester and John Winchester respectively.
Every season (except the first) has an episode where the narrator is somebody other than Meredith (this time is Izzie). However, this is the first season with more than one of those episodes (Denny narrated "Stairway To Heaven", and Alex narrated "Elevator Love Letter").
Every episode of the show is named after a famous song. "What a Difference a Day Makes" is a song by Dinah Washington.