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 days 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.
Note: The calendars on these pages are shown as the moon would appear when viewed by people in the northern hemisphere. If you are viewing the moon from the southern hemisphere, it would be illuminated from the other side (i.e. the waning moon would be illuminated on the RIGHT.) |
Full Moon 
New Moon 