From the Rebbe’s explanation:
– What is the key difference between the weekly cycle and the monthly cycle in the Jewish calendar? The weekly cycle reflects a Divine pattern of revelation, paralleling the first seven days of Creation. It’s determined by the sun’s movement and not dependent on man’s activity at all. The holiness of Shabbat was established by G‑d during the first seven days of Creation and it’s above all connection to human actions. From the seventh day of creation onward, every Shabbat has been an experience of holiness by the Divine decree.
In contrast, the monthly cycle is established by the Jewish. It’s dependent on man! In fact, the very first mitzvah commanded to the Jewish people was to sanctify the new months and set the calendar.
This is reflected in the blessing recited on holidays, “…who sanctifies Israel and festive seasons.” Israel is mentioned first, for the sanctity of the festivals is dependent on the Jewish court. This points to the spiritual task given to the Jewish people, to draw down holiness which transcends the creation within our world…
These two levels relate to the numbers seven and ten. Seven relates to the holiness of the natural order, i.e., the Divine life-force invested in creation. Ten, in contrast, relates to a level which transcends creation and is introduced by man.
To explain the concept in Kabbalistic terminology. Seven reflects the seven middos, the Divine energies which are paralleled by our emotional qualities. These were the forces which brought the world into being; each day of creation giving expression to a different one of these middos (the first day of creation reflected the quality of Chessed, “kindness,” the second day, the quality of Gevurah, “might,” until the seventh day which reflected the quality of Malchus, “sovereignty”).
Ten, in contrast, also includes the three qualities described as mochin, which are paralleled by our intellectual faculties. These qualities transcend the present level of the world. The intent, however, is for man to introduce these intellectual qualities into the world, and thus to elevate the world to a higher plane.