The Sun enters Libra on September 22nd, kicking off the Fall Equinox. For this day only, the hours of daylight and darkness will be balanced. Then, we tip over into longer nights.
There's two things happening here. The moment of equilibrium, when one side is balanced against the other. And the acceptance that the season of light/growth is ending, and the season of dark/sleep is beginning. This is what Libra is really all about; an acceptance of light and darkness (not just focusing on the pretty stuff and ignoring the rest). There is no "good" or "bad" (no matter how you may feel about incoming winter). The season of cold dormancy is necessary; everything needs to rest.
If you want to use the Fall Equinox's energy, this would be a good day to take stock of where you've been/what you've done (this year) and where you want to go. How do the two sides balance out? How have you handled growth and retreat? The trick is knowing when to start, and when to pull back. It's a fine balance.