Crystals have the ability absorb and hold unique energies, both positive and negative. So its important to cleanse them often and the full moon is the perfect time to do this. When we say "cleanse" in this context, it's on a energetic level, not just cleaning the dust off them. Many people believe that the full moon can clear a crystal's energy and recharge the positive energies it holds.
On the full moon, set your crystals outdoors or on a windowsill where it will be bathed in moonlight, and leave them out until morning. The moon cycle is a great regular reminder to keep your stones neat and tidy. Approximate time: 8-12 hours.
I'm sure you have lots of questions about cleansing and re-tuning your crystals with full moon energy. Why you don't check this complete guide from Crystal Guidance. Other than full moon, there are also many other ways to cleanse the crystals. Check this post to find out more, as well as the list of crystals that don't need cleansing, crystals that don't like water and sunlight.
P.S. Here's my Crystals Love Moonlight blog post recapping from Crystal Guidance.