Months are about 30 days long. Years about 360. But weeks? Well, they are 7 days long. And so they don't fit evenly into either months or years. Within the calendar, they are kind of the odd one out. The week cycle doesn't nest within or coincide with the months, seasons, or other cycles in any way.
This is something that I've often contemplated, and wanted to change. We could redesign the calendar so that each month had 28 days (4 * 7), or 35 days (5 * 7). Then there we could pad the end of the year with an extra "Saturday" (or two, on leap years), and every month would start on a Sunday. Every 3rd would be a Tuesday, and every 9th a Monday. Your birthday would always be on the same day of the week.
The weeks would finally be in sync.
But yesterday, as I was thinking about this, I was considering where the week comes from. Unlike the other elements of the calendar, it isn't based on a natural cycle. The month is based on the phases of the moon, the seasons of the year on the tilt of the earth, and the year on earth's orbit around the sun.
But the week doesn't coincide with any of these, or any other natural cycle. We don't have weeks in our calendar because of some cosmic phenomenon. But rather, because of a supernatural phenomenon: the seven days that God took to create the world (Genesis 1).
Weeks are designed by God to be non-conformant. They remind us that it isn't just celestial cycles that rule our lives and the history of earth. There is also a supernatural element, that is imposed upon creation, and not derived from it. The natural cycles are not all that there is. The history of the earth also revolves upon events which are not mere consequences of the laws of physics.
The week should thus not only remind us of the earth's supernatural origin, but also of its supernatural destiny. As Peter said, "One day is with the Lord as a thousand years, and a thousand years as one day." The seventh day, when God rested, points to the rest that we will one day enter into with God (Hebrews 4).
So the weeks are discordant from the rest of the calendar for a reason. God chose a 7 day week, that would not fit with the natural cycles, to remind us that it isn't just matter and energy that govern the world, that created it, or that determine its end. It is an all-wise and loving Creator, who created all things, by whom all things consist, and who has prepared a rest for his saints.