Book 1 of the Birthmarked Destiny Series:

Once a century, a pony's destiny is chosen by a mark on her cheek.

Twilight is the chosen one to bear the mark of her destiny and she is done with sitting in the castle all day, every day for the rest of her life.

After meeting with Celestia Twilight is given a chance to be free. In order for her to remain free she must prove herself by blending in with society by helping out with the Summer Sun Celebration.

As the time gets near a villain who has been waiting to seek revenge has returned to wreak havoc on Ponyville and the citizens of Equestria.

Cover not done by me. Cover done by Vexx3 on deviant art, not by me. I only added the title display.