Night of Aeolus

The Night of Aeolus is first mentioned in the Charmed Novel, The Brewing Storm by Paul Ruditis.

About
The Night of Aeolus takes place every hundred years, whereby four Elementals, i.e. a Firestarter, a Waterbearer, an Earthshaker and a Windwalker, come together to perform a ritual in the Circle of Gaea. This ritual is to calm the turmoil that the world undergoes every century. The Circle of Gaea changes place every century though there is one similarity in all the locations: they are all in nature and have a pentagram. Also, to perform the ritual, a wand, a sword, a chalice and some coins need to be handed out to the Elementals; the sword and the wand are interchangeable between Air and Fire, the chalice to Water and the coins to Earth. To fill in the fifth spot on the pentagram, a Spirit Elemental is required. A Spirit Elemental is a pure human who is not "suppressed by magical powers".

In The Book of Shadows
Once every century, the planets align in such a way that it causes the winds to shift and the climate to change.

During that period, weather becomes unstable and the Earth begins to shake as the world falls out of balance.

On the Night of Aeolus, by the light of the full moon, the elements must come together to set everything in its proper place.

To do this, a Firestarter is charged with gathering other Elementals at the Circle of Gaea to perform the Ritual of the Guardians.