I think those are travel heavy days, but June through August are travel heavy months.
That and the fact that you've got many families with younger children who will stay home or only travel nearby for the majority of the holidays, whereas they'll have a family trip planned to a location that is further away for the summer months. Probably the same with older people...they'll get out more in the summer than in the winter, even though two major holidays are in winter.
Also, there's a seasonal gasoline transition between summer-grade fuel and winter-grade fuel. The difference is in the additives used. Summer-grade fuel is always slightly more expensive than winter-grade fuel due to the price of the ingredients, as well as the fact that the refineries have to have a brief shutdown before being able to process summer-grade fuel.