Ethanol production is present all year round in the US, however, certain seasons record its peak production.
Generally, the corn harvest in the US runs from late summer to fall. During this time ethanol production is plenty and prices are low as there is ample supply yet the summer driving season from Memorial Day to Labor Day causes a short spike in price.
Furthermore, the Clean Air Act in the US requires much tighter regulation to lessen smog and other air pollutants. This law takes effect every June 1st to September 15th.
The state of Minnesota prohibits selling ethanol during the summer months to fully follow the regulation. Regardless of this law, ethanol production, supply, and demand continue across the state.