Why don't the phases of the Moon occur on the same day each month?
The different phases of the Moon do not happen on the same day each month, because the Moon's orbit around the Earth does not take exactly one Month. The cycle of the phases from new Moon to new Moon takes 29 and 1/2 days. Because of this, Moon phases can happen anywhere during the months.
 

January | February | March | April | May | June | July
August | September | October | November | December