The Thames had frozen over either wholly or in part at various times before 1683, and leisure activities had taken place there. But in December of that year the Thames froze for two whole months, prompting what some social historians believe to be the first official Frost Fair, rather than a spontaneous gathering on the ice.
The website, Where Thames Smooth Waters Glide (2) gives a contemporary explanation for the emergence of the Frost Fair. It records (from a pamphlet in the British Museum) that the Thames was so blocked with ice that the watermen were unable to ply their trade, carrying goods and passengers up and down the Thames.
To compensate for loss of earnings watermen took to the ice, setting up stalls to sell liquor and food to an eager public. Eventually roads were built on the ice and carriages began to ply their trade up and down the frozen river.
Soon the phenomenon was being called a Frost Fair, because the crowds were so great and jovial, just like those at the annual Bartholomew’s Fair in Smithfield.