The moon can be measured by various cycles.
The 28-day count is derived from the mean (average) lunar cycle.
the
synodical cycle, from new moon to new moon, is 29.5 days
(what most lunar calendars are based on)
the sidereal cycle, the measure taken from where the moon appears at
the same place in the sky, is 27.33 days
there
is also an apsidal cycle, taken from the measure of when the moon's
axis is tilted farthest from the Earth, which is 28 days.
Counting
the year by 28-day moons, we have:
13 X 28 (+) 1 day out of time = 365 days - a perfect formula!
The 28-day cycle is also the average of the female fertility cycle, oftened referred to as one's "moon cycle."