It was Twilight Sparkle coronation day, to celebrate her being the new Princess of Equestria.
It was supposed to a day to remember. Indeed it was. If ponies were to remember that their newly founded Princess were to be suck in a mysterious portal and then to disappear and never to be seen again. Then every pony seem to have remembered that day.

*set after the finale of season 3*
Twilight Sparkle was just in front of hundreds of ponies giving her speech along side Princess Luna and her mentor, Princess Celestia. The next moment, she is suck through a portal to... well who knows who.
Only Twilight Sparkle will be able to tell the tale.