The week — and existence at large — is based on a cycle of seven. Each week is made up of six week days that culminate in the Shabbat, and the universe is comprised of a cycle of six thousand years of existence as we know it which will culminate in (the seventh millennium of) the World to Come. That being so, we’ll delve into the Shabbat before we examine the Holy Days and other singular times in the Jewish calendar, and we’ll see just how pivotal the Shabbat is to the workings of the universe.
(There are a number of reasons offered as to why the number seven plays such a significant role, such as the uncanny fact that there are seven notes to the tonal musical scale; or the idea that the number six is the sum of a perfectly simple and elegant progression of one plus two plus three, which then starts all over again with another number one, adding up to seven, etc. At bottom, though, we’re taught that there’s actually no more reason to figure out why seven is basic to the universe than it is to try to figure out why we have two eyes rather than three or the like; for we’re to trust that G-d has His reasons for doing things the way He does, and to know that each and every phenomenon plays its own imperative role in G-d’s intentions for creation.)
There’s another essential phenomenon unique to the Shabbat aside from it being the distinctive seventh day of the week, having to do with the following. We learned earlier on that there are a plethora of both holy and non-holy (i.e., profane) elements throughout the universe in keeping with G-d’s plans (see 4:6:1). So there had to be a way for humankind to not be overcome by so much non-holiness.
G-d thus saw to it that there’d be a just-so amount of non-holiness to contend with, and that there’d be an escape-hatch to holiness, if you will. We can’t fathom the ramifications of it, but we’re to know that a lot of Divine attention was paid on the determination of just the right mix of holy and profane (both how much of each and of what sort was to exist; when and where each quality would dominate, etc.) as well as on a determination of just how far the combination could go before there’d be just too much of the profane.
It was determined that there had to be a just-so combination of holiness and profane in the course of *the week* as well, and G-d resolved that the best combination would be the six-parts profane and one-part pure holiness cited above, and that the Shabbat was to provide the one-part pure holiness.
It was also decreed that time was to consist of a rotating cycle of seven days rather than an unending series of days (which also helps explain why the universe was created in seven days specifically), and that each element of the universe was to fall under a cyclical paradigm. It was even determined that history as we know it would end in the seventh, Sabbatical millennium “of rest” then begin anew (on a wholly other level that’s actually above time and beyond our ken …. but that’s a subject unto itself). It was likewise determined that each “seventh”, the Shabbat and The World to Come, was to be holy and capable of elevating the preceding six (making all of existence holy in the end).
It’s clear then that our having been granted the Shabbat (as well as The World to Come) was a great gift from G-d who wants us to be a holy nation and has bidden us to elevate the universe and sanctify it.