So the month always has the same number of days, and there are sometimes 7 days from Sabbath to Sabbath, and sometimes 8?
Where did you get this from? I thought maybe this was a Qumran sect, but that doesn't seem right . This doesn't have anything at all to do with what Jews today do, so you'd have to explain why and when this changed. Not to mention the fact that the Bible talks of the 7th day- but your 7th day is sometimes the 8th. And the fixed month length will very quickly be out of touch with the real seasons, so your "Aviv" (and Passover) will end up in the wrong season.
In short, this seems weird, to put it mildly. So please explain where it comes from.
The pagan Roman Julian/Gregorian calendar that we now use was only created in 45 BC, before this time it did not exist. Therefore, the current 7 day rolling week we currently use only began at that time. Before this date the Romans (Just in the Provence of Rome) were using a calendar with an 8 day week:
https://en.wikipedia.org/wiki/Roman_calendar
The Greek Mathematician Sosigenes who Julius Caesar employed to create the new calendar simply removed one of these days (Market day), so the chances of the current 7 day week matching the Jewish week was only a 1 in 7 chance.
The calendar Jesus and the Israelites had been using in Israel for over 1500 years prior to this time was completely different to the Julian calendar/Gregorian calendar. Their months began with the sighting of the new crescent moon on which they rested and had a new moon feast, then there were 6 days of work followed by the first Sabbath. Lunar months are 29.53 days long and therefore Sabbaths fall on day’s 8, 15, 22 and 29 each lunar month. Then the process begins over again with the sighting of the next new moon.
However, God’s Calendar is not the same calendar the Jews of today use. The Jews currently use a revised version of God’s Calendar which uses the Gregorian rolling week system instead of God’s lunar weeks, this was due partly to the Roman Emperor Hadrian after he had put down the Jewish Simon Bar-Kokhba revolt in 135 AD and made an anti-religious decree forbidding Sabbath new moon observances from Jerusalem and also the Roman Emperor Constantine who began to force the Jews in 325 AD to worship on the pagan day called Saturday (named after the Roman god of agriculture; Saturnus).
The Jewish calendar used today was created in 359 AD by Sanhedrin Rabbi Hillel II, this was in response to the Romans banishing the Jews from living in Jerusalem, as the Jews could no longer sight the new moon from there and so Hillel’s calendar uses a repeating 19 year lunar mathematical calculation that approximates the timing of each new moon sighting from Jerusalem. This allowed all the Jews to celebrate their feasts on the same day no matter where they had been scattered all over the Earth.
The fourth commandment; ‘Remember the Sabbath and keep it holy,’ has virtually been forgotten, a command more important than murder for example, which is the sixth commandment. Although we are saved by grace we should still use the ten commandments as our guide as to how to live. We are not dependant on the law but it will prosper us if we adhere to it, equally if we break these laws then we will face the consequences.
I hope this helps you to understand that the Sabbath is not on Saturdays or Sundays (Which are all pagan names that were invented long after God's Calendar of the Bible) but instead can fall on any day of the pagan Julian/Gregorian week. The calendar the Jews used was written about by Moses in the Bible, it is God’s Calendar and you can read more about how it is explained in scripture:
Robert Nicholas - The Christian's guide to the Bible:
https://guidetothebible.wordpress.com/start-reading-the-book/
John D Keyser – founder of Hope of Israel ministries:
http://www.hope-of-israel.org/sabtosat.htm
Further studies on God’s Lunar Solar Calendar by other Bible scholars:
International Date Line Change: The Sabbath Unchanged by Worlds Last Chance Ministries?
https://www.youtube.com/watch?v=7sUyZpM5xls
Robert-Aaron Richmond explains from scripture how God’s Calendar works:
https://www.youtube.com/watch?v=wJq8tKvnemU