That's important because it reconciles the solar and lunar cycles, which is a key feature of the Jewish calendar. And the Sabbath is counted every seven days, which is a function of the human mind and of the community - that's the third element of the Hebrew calendar. So Shabbat ha-Gadol is "big" because it ties together these three elements and anchors the whole yearly cycle.
We've got beautiful weather here in Portland and I'm just taking it easy today. Sunday I'll get back to my spring cleaning.