How often must congress meet? why?

They have to meet at least once a year. It ensures that the congress continues meeting and upholds democracy. In Britain, Parliament couldn't meet unless the king called the meeting.

the declaration states, “we hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness….”