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Unfortunately, the lunar month is not an exact number of days or weeks. It is currently about 29.5306 days. So was it ever 28 days (4 weeks of 7 days) sometime in the past?
Since ocean tides sap orbital energy from the moon, then surely the moon should be gradually falling towards the earth and hence speeding up. In other words, the month would become shorter. Furthermore, the Earth's rate of rotation is decelerating at 1.4 milliseconds per day per century or thereabouts. Today is always a little bit longer than yesterday and tomorrow will be a little bit longer than today. This would tend to make the month appear to get shorter even faster. This would mean that the month could never have been exactly 28 days in the past. If at all, it can only become 28 days at some time in the far distant future.
This notwithstanding, it is unlikely that the number of days in a month has changed significantly while sentient humans have lived on this planet. I am no astronomer, so if you know the answer, please email me.
This at first seems to throw doubt on the idea that the 7 day week came from moon phases observed by the Ancients as some suggest. But that is only if we assume that weeks are concatenated blocks of 7 days - that is, if we assume that a week always begins with the day immediately following the last day of the previous week.
Some ancient lunar calendars used a 7 day week with allowed for 'frame slippage'. That is, when the week got more than a day out of synchrony with the moon's phases, you simply inserted a 'padding out' day (or sometimes two days) at the end of a month to delay the start of the first week of the next month. I am given to understand that such a day was referred to as a Dark Moon day because on such a day the moon would not be visible. A Dark Moon day was not a part of any week. It was outside on its own.
The 7 day week has been maintained through millennia as a unit of time of exactly 7 integral days. But a calendar of continuous concatenated integral 7 day weeks has no natural frame of reference with which its synchrony could be scientifically verified at any time in history or any place in the world. It is totally dependent on human counting or time-keeping by some central human authority or college. The human propensity for mistakes makes it very doubtful that this counting could have continued over thousands of years through all humanity's migrations, wars, social changes, religious upheavals and intellectual differences without error.
Some religious people think the lunar-week (where you pad out the month to maintain lunar synchrony) is correct. Others think the fixed-week (where you just keep on counting 1 through 7 then start immediately at 1 again) is correct. The main contention this causes among people of certain religions is that it effectively changes the day on which their respective Sabbaths fall.
The 7 annual holy days mentioned in the Bible form a fixed pattern of timing from the first holy day to the last holy day. However, this pattern is far from synchronized with the solar year. Its position within the solar year jitters from one year to the next. Or as engineers would say, it suffers frame slippage. Hence these holy days shift slightly from year to year in terms of the solar or botanical seasons in which they fall. Furthermore, the number of days between the last holy day of one year and the first holy day of the next year can vary enormously. I don't think the asynchrony of the annual holy days within the solar year is anywhere disputed.
I speculate that the annual holy days were made asynchronous deliberately: firstly because they represent the phases of a once-only transient process, and secondly because the times between the phases are not pre-set, but depend on when certain predicted circumstances materialise within the complex dynamic of human society.
The 7-day week also is a repeated or cyclic reminder of a once-only or transient master plan. So as the annual holy days bear an asynchronous relationship to the annual cycle, so too would I expect the weekly holy day to bear an asynchronous relationship to the lunar cycle. And for the same reason - to ensure that we realise that the process we are being repeatedly reminded of is a once-only sequence of events rather than a recurring sequence of events.
The imperfect fit between the daily, lunar and annual cycles does not sit comfortably within the minds of mechanistic thinkers. The Romans seemed to have a problem with it. I think the 'clockwork universe' Victorians would have too. I did. That is, until a few years ago when I introduced myself to chaos theory. Now I welcome the assynchrony between the earth, moon and sun. It writes for each of us a unique and inspiring melody upon the regular rhythm of life, making each day, month, season and year a unique and memorable experience.
Moon Phases
For details on the moon's phases and how they vary from day to day, look at some of the following sites.