Equinox

An Equinox occurs when the sun passes directly over the equator. It occurs twice every year at the beginning of spring, and fall. It is a powerful day in and of itself, the convergence of powers is even stronger, the potential greater. All one must do is tap into it.

The Charmed Ones' Anniversary
In 1999, the Charmed Ones celebrated their one year anniversary of becoming witches. According to a friend of Phoebe's, Stevie, their anniversary fell on the Equinox.

This would make them able to connect with the Equinox and enhance their powers as well as their universal bond, the Power of Three.