While showing nearly the same face to Moon spins for its day and night. As of now average Moon’s day is 29.530587891 Earth days.
Here the period between two consecutive Full Moon’s transits (or New Moon’s transits) of the Earth’s heliocentric ( revolving-about-Sun ) longitutudinal plane determines lunar day.
In brief it is synodic (defined with respect [the Sun]) lunar month (http://socratic.org/astronomy/our-solar-system/the-sun-1)-Earth line.
Moon spins through 2##pi## radians with respect to the Sun during this period.