  The signals in dark matter direct-detection experiments should exhibit
modulation signatures due to the Earth's motion with respect to the Galactic
dark matter halo. The annual and daily modulations, due to the Earth's
revolution about the Sun and rotation about its own axis, have been explored
previously. Monthly modulation is another such feature present in direct
detection signals, and provides a nearly model-independent method of
distinguishing dark matter signal events from background. We study here monthly
modulations in detail for both WIMP and WISP dark matter searches, examining
both the effect of the motion of the Earth about the Earth-Moon barycenter and
the gravitational focusing due to the Moon. For WIMP searches, we calculate the
monthly modulation of the count rate and show the effects are too small to be
observed in the foreseeable future. For WISP dark matter experiments, we show
that the photons generated by WISP to photon conversion have frequencies which
undergo a monthly modulating shift which is detectable with current technology
and which cannot in general be neglected in high resolution WISP searches.
