John Hale of Dunedin asks :-
If a lunar month is 28 days, does that mean a full moon is always on the same day of the week?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
A lunar month isn't 28 days long. The average time from New Moon to New Moon is 29.53 days. So the day of week of a particular moon phase progresses though the week.
The average calendar month is 30.4 days long, or 30.5 days in a Leap Year. So a lunar month is nearly one day shorter than the average calendar month. That causes the date of New Moon to shift backwards through the months.
Because February has 28 days usually, and the other months have 30 or 31 days, the shift isn't smooth. Through the 12 months of 2013 the days and dates of New Moon are: Saturday 12, Sunday 10, Tuesday 12, Wednesday 10, Friday 10, Sunday 9, Monday 8, Wednesday 7, Thursday 5, Saturday 5, Monday 4, Tuesday 3. The other moon phases change similarly. Here 'New Moon' is the time when the moon is in line with the sun, the way astronomers define it. We don't actually see the thin crescent moon till a day or two later.
Many cultures use lunar-based calendars that work from New Moon to New Moon. Twelve lunar months are 354.4 days, 11 days short of a solar year. Some of these cultures add extra days or extra months from time to time to keep months roughly in line with the seasons. Some don't. It's an interesting topic in itself, its history in particular.
Because the earth orbits the sun the moon has to do a complete circuit of the earth and a bit more to get in line with the sun again. The time taken for the moon to orbit as seen from a fixed point in space is 27.32 days. Astronomers define five different 'lunar months' depending on which aspect of the moon's orbit is being discussed.