The Summer Sun Celebration commemorates the summer solstice. Ponies everywhere throw raucous parties the previous night and stay up until dawn to watch Princess Celestia raise the sun. Celestia effecting sunrise isn't a special or odd occassion in Equestria - she does it every morning. This is simply the one day of the year she makes a spectacle of the event.
It is on this day that a legend foretold the release of Nightmare Moon from her millenial imprisonment within the moon. It came to pass in the first episode, Mare in the Moon. From what is shown in the second episode, Elements of Harmony, it is assumed the sun rose normally on that particular solstice. Celestia was busy consoling and bonding with her penitent sister.