The 100th episode to the My Little Pony: Friendship Is Magic series
Dr Whooves is a main character in this episode. His character is based on Doctor Who's main character the Doctor. He even uses the Tenth Doctor's catchphrase "Allons-y!" and wears the Fourth Doctor's scarf at one point.
Twilight Sparkle and Rainbow Dash are the only members of the "Mane 6" to have speaking roles.
Sunset Shimmer was going to have a small cameo, but it was cut because it made no canonical sense. Likewise, Trixie was also to appear, but her part was cut as "she isn't exactly allowed in Ponyville, and wouldn't have any reason to attend 'some mule's wedding'."
The three ponies first met in the bowling alley are references to the three main characters in the movie The Big Lebowski.