Fly by night is a once-popular (1950s-1960s), derisive term applied to travelling businessmen and tradesmen. It implies that the quality of work done or goods sold is shoddy enough that the perpetrator must move from town to town each night to evade responsibility.
In actual fact many travelling tradesmen perform exemplary work, and travel from place to place trying to find areas that are underserved by their specialty or that are experiencing rapid enough growth that local tradesmen cannot keep up.
See Irish rovers in America for one ethnic group where travelling tradeswork is unusually common.
Fly by Night is the name of an album by the popular music group Rush.