where did be there or be square come from

The exact origin of “be there or be square” is a bit murky. There are two main theories:

1940s Jazz Scene: The most widely accepted theory suggests the phrase emerged in the 1940s jazz scene. Back then, “square” referred to someone considered boring and out of touch with current trends, especially those who disliked jazz music. So, “be there or be square” meant “attend the jazz event or be seen as uncool.”