Park, San Francisco
The Charmed Ones connect with the Equinox, vanquishing the demon Abraxas.
An Equinox occurs when the sun passes directly over the equator. It occurs twice a 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 has to do is connect.
The Charmed Ones' Anniversary
This would make them able to connect with the Equinox and enhance their powers as well as their universal bond, the Power of Three.