[Image taken from here]
Happy Moon Day! May you have a mind like the moon!
Wait... why would anybody want to have a mind like the moon? You may be thinking. Why would anybody want a mind that is full on certain days, empty on others, and half-full on yet others?
But this is not what I have in mind (no pun intended). "Mind Like the Moon" is an expression in the Japanese martial arts (Tsuki no Kokoro, in Japanese). The idea is that when facing an opponent in combat, one's mind needs to be like the moon, calmly watching over everything in the environment in the present moment, so as to be able to react instantly and appropriately to whatever is being thrown at one.
This concept applies not just to combat, but also to everyday life. The idea is that if one is fully present in the moment, one will be able to calmly observe everything in one's environment, and respond and interact with the environment accordingly.
You can probably see the parallel with yoga practice here. In our practice both on and off the mat, we work on cultivating a state of full awareness of everything; things in the environment, as well as sensations within our bodies and the endless chitta-vrtti-feed that is our consciousness. In this sense, I suppose we can say that our minds become moons that shine on things both within and outside us.
Enjoy your moon day, and may you cultivate a moon-like mind :-)