Which season 5 episode was Grey's Anatomy's 100th overall episode?

“What a Difference a Day Makes” was Grey’s Anatomy’s season 5 episode that became the series’ 100th episode overall.

It is the 22nd episode of season 5, written by series creator Shonda Rhimes and directed by Rob Corn. ABC first broadcast it in the United States on May 7, 2009. The milestone episode centers on a wedding, while several characters confront major emotional turning points.

The title can be confused with nearby season 5 episodes such as “Now or Never,” the season premiere, or “Elevator Love Letter,” which is season 5’s 19th episode. “Dream a Little Dream of Me” is also from season 5, but it is the opening half of a two-part premiere and not the centennial installment.

The episode drew about 15.326 million viewers in its original broadcast and was praised particularly for performances by Katherine Heigl and Justin Chambers. Jeffrey Dean Morgan also returned as Denny Duquette in a final appearance.