This paper investigates the economics of "blue laws" or restrictions on shop-opening hours, most commonly imposed on Sunday trading. We show that in the presence of communal leisure or "ruinous competition" externalities, retail regulations can have real effects in a simple general equilibrium model. We look for these effects in a panel of US states and in individual CPS data. An instrumental variable approach identifies the effects to the extent that blue laws are endogenous. We find that blue laws 1) reduce relative employment, 2) raise annual compensation and labor productivity, and 3) depress prices in the retail sector. The primary mechanism appears to be a reduction of part-time employment.