There is every reason to believe the pattern at the state level will be repeated if President Barack Obama is re-elected and the Democrats retain control of the Senate.
"For whatever reason, there's a predictable pattern where September has bad performance, " says Steven DeSanctis a strategist with Bank of America Merrill Lynch's equity research team.
If anyone has research explaining whether there is a true pattern of more sicknesses on Sundays and a medical reason for it, I would certainly welcome the information.
One reason is that seasonal adjustment is tricky in such countries where the shift from agriculture to industry changes the pattern of seasonality over time, says Mr Kroeber.