Below is a sneak preview of Thursday's episode of Grey's Anatomy - the show's 100th - "What a Difference a Day Makes." Mark and Meredith chat about the wedding in surgery, and he "warns" her that this is the second time he's been Derek's best man ...

[video url="http://www.greysanatomyinsider.com/videos/what-a-difference-a-day-makes-sneak-preview-3/" title="What a Difference a Day Makes Sneak Preview #3"][/video]

Besides the big wedding, what else is happening at Seattle Grace this week? Follow the link for some additional clips from the episode and more Grey's Anatomy spoilers ...

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.