phase of the moon: n.

Used humorously as a random parameter on which something is said to depend.
Sometimes implies unreliability of whatever is dependent, or that reliability
seems to be dependent on conditions nobody has been able to determine.

"This feature depends on having the channel open in mumble mode, having the foo switch set, and on the phase of the moon."

There are eight moon phases: and back to the new moon. The United States Naval Observatory, of course, explains it in much more detail.

You can learn why I chose "Phase of the Moon" as a domain name by reading this Jargon entry.