These are the things that the Eternal has commanded you to do: On six days work may be done, but on the seventh day you have a Sabbath of complete rest, holy to the Eternal, whoever does any work on it shall be put to death. You shall kindle no fire throughout your settlements on the sabbath day. (Exodus 35:1-3)
Shabbat is extremely important to the Jewish people. One of the founders of cultural Zionism, Ahad Ha'am, is often quoted as saying: More than the Jewish people have kept Shabbat, Shabbat has kept the Jewish people.
We know that Shabbat is important; the 4th commandment is entirely devoted to its observance. We also know that it is a day of rest; there can be no work on Shabbat. But this requires a bit of semantic unpacking, as our colloquial definition of "work" and "rest" do not necessarily translate exactly to the underpinnings of Shabbat.
In this section of portions in the Torah, God commands the building of the tabernacle. It turns out that there are 39 actions that God instructs. The Israelites paint, sculpt. They light fires, they hammer and nail ... If we made a spreadsheet consisting of each instruction to the Israelites, we would see 39 verbs repeated over and over. Taken together, these 39 actions are exactly what is considered avodah - work.
This explains why you can walk 5 miles in the hot sun to synagogue on Shabbat, but you can't drive, because driving is akin to "lighting a fire." Similarly, one can't watch TV, but you can read a book. I'll put it this way: On Shabbat, Jewish law prohibits us from performing the exact 39 actions that were done when constructing the tabernacle and the mishkan.
Somewhat ironically, it actually takes quite a lot of work to celebrate Shabbat. It isn't a day of laziness. It is meant to be a day of intention and of serious engagement and learning. It is also meant to be different than the other days of the week. During the week, we build. On Shabbat, we reflect and consider. During the rest of the week, we petition God for those things lacking in our lives. On Shabbat, we only show gratitude and joy.
Why do we celebrate Shabbat?
One answer comes easily. We rest on the seventh day because God rested on the seventh day. After God's 'work-week' of the 6 days of creation, God stopped creating in order to reflect upon creation. Shabbat becomes an example of imitateo dei - the imitation of God. This point is emphasized again when the Israelites receive the Ten Commandments: Remember the Sabbath day, to keep it holy ... For in six days God made the heaven and earth...
But as you may know, the 10 commandments are repeated again in the book of Deuteronomy. But here, the reason for our Shabbat does not have anything to do with creation: Remember that you were slaves in Egypt and that the Lord your God brought you out of there with a mighty hand and an outstretched arm.
For this one holiday, Shabbat, the Torah gives us two very important reasons for its observance.
We celebrate Shabbat because of our connection to God. Because we are created in the image of God, we are meant to emulate God's holiness: God creates ... we create. God is holy ... we are holy. God rests on the seventh day ... we rest on the seventh day.
But we also celebrate Shabbat because of history. The commandment in Deuteronomy teaches us that Shabbat connects us to events in our past. Shabbat helps to reconnect us with God, but also with each other, as we remember the miracles that God has performed for our ancestors, in our history ... and in our own lives.
We keep Shabbat so that we can rest and have a day that's different than the other days of the week. But we also keep Shabbat so that we stay connected to our tradition, our God and ourselves.