A great deal happened to the Jewish people during the seven weeks between the Exodus and the giving of the Torah on Mount Sinai.
They fought a war with Amalek, they complained about food and water, and received the Manna.
When they began their journey from the parted Red Sea, they traveled to the desert of Shur. The Torah tells us that they traveled for three days, and could not find water.
The symbolism of the word, “מים,” water, is that it could be a reference to Torah. Just as water gives sustenance, and without it, we cannot survive, the same is true in connection to the Torah.
Based on this Pasuk, Ezra made a decree that Jews should not go three days without Torah. He instituted the practice of reading the Torah every Monday and Thursday. Torah is our life force, and cleaving to it, gives us strength. The Torah and Hashem are one. This is our special destiny in the world. When there is a connection to Torah, the world becomes a better place.
There was a cute sign at a Chabad Shule in Los Angeles that said something similar. “Seven days without Torah, makes one weak.” This is why Ezra felt that we need to get changed up every Monday and Thursday.