Much will be said today, much of it patriotic sentimental blather about liberty and freedom and the moral virtues of limited government.
The most important fact about the Constitution, indeed the hermeneutical key to reading it, is this: The Constitution was written to create the strongest possible Federal Government that would not have, and could not ever have, enough power to end slavery.
Even the second amendment, which is at heart, the right of states to have state militias. If the main force of men in arms in the early republic was to become the United States Army, under the command of the President, could southerners count on it to protect them from slave uprisings? Could they require white men to join the 'slave patrols' which were the local forces of slave repression?
Compromise after compromise was fashioned to keep the southern slave-owners in the coalition for a new Constitution, while avoiding permanently affirming the institution of slavery. It was a successful effort -- the only way that slavery could be abolished was to go outside of the Constitution, militarily defeat the southern states, and change the Constitution while they were in rebellion.
There is nothing sacred about the Constitution of the United States. It was an ingenious and clever act of political craft. It created a government which is stable and sustained order for all these years, if you don't count 1860-1865. But it leaves in place so many obstacles to democracy, so many institutional barriers to the people working their will through the government to improve their conditions of life, that it has also impeded social progress all these years as well.