What is so luminous about a day? What is so precious to captivate the hearts? It is because the seventh day is a mine where spirit's precious metal can be found with which to construct the palace in time, a dimension in which the human is at home with the divine; a dimension in which man aspires to approach the likeness of the divine.For where shall the likeness of God be found? There is no quality that space has in common with the essence of God. There is not enough freedom on the top of the mountain; there is not enough glory in the silence of the sea. Yet the likeness of God can be found in time, which is eternity in disguise.
Bonus -- this isn't Heschel, but a beautiful sweet 90-second teaching about Shabbat from Rabbi Nehemia Polen