Because the calendar is based on the lunar cycle, every full moon will be the middle or the 15th of a lunar month, which is either 29 or 30 days long.
(the calendar; is based; on the lunar cycle) (every full moon; will be; the middle or the 15th of a lunar month) (a lunar month; is; either 29 or 30 days long)
the calendar every full moon a lunar month
is based will be is
on the lunar cycle
the middle or the 15th of a lunar month
either 29 or 30 days long