Solar Eclipse

A Solar Eclipse is an astronomical event where the moon passes between the earth and the sun, hence (partially) blocking the sun. Any and all curses may be broke and the Elders are unable look down on earth from the Heavens.

History
A demon called "the Boss" cursed two people by the names of Brooke and Christopher, he told Brooke that the only way to break the curse was to give in to his demands or when there would be "a night within a day". Phoebe Halliwell then realized that this meant the spell could be broken during an eclipse. Along with Christopher, the sisters were able to keep Brooke from giving in. Brooke and Christopher then kissed and broke the curse, and the Boss was vanquished.

Meanwhile, Phoebe also hypothesized that during the eclipse, the Elders weren't able to look down, as people on earth couldn't look up. During the eclipse, a secret ceremony between Piper and Leo was hurried to take place; unfortunately Cole (being the evil that he is) told the Triad of the sisters and Leo's plans, and they (being the evil they are) shared their knowledge with the Elders...and they took Leo away.

Notes and Trivia

 * According to the Handfasting entry in the Book of Shadows, the ceremony can be performed best when both the sun and the moon are present, which includes an eclipse.