There is an etymology word joke that says something along the lines of, "if "pro" is the opposite of "con", then is the opposite of "congress" "progress"?"
And if you don't know etymology, then that seems to make sense.
When you break down the word Congress, you get the prefix con and the root word gress, con means with, and gress means step, so it means to step with or to walk with.
The opposite of walking with someone is to walk apart from someone, so, the actual opposite of congress would be digress, and the opposite of progress would be regress.
Etymology is great at ruining jokes, but it's also great at helping you understand what words mean and why they mean them.