They mention some important ideas regarding summer as a time for increased attacks like increased flow at transportation hubs, weather, etc.
It’s interesting that they did not go into the history much. The Taliban who harbored and in some cases became AQ had a history of doing most of their dirty work in the summer and retreating into the mountains during the winter. In Iraq, we have seen stepped up attacks during the summer. In Israel, Lebanon, and surrounds things tend to have a history of ratcheting up during the summer. These are things that should not be ignored either as I think it places significant emphasis on there being an increased threat during that time period. It’s not to say we should let our gaurd down at other times, but the summer months seem to be the ones for big offensives worldwide.
On a little bit lighter note...
One night (a year or so ago) I was cyber touring and ran across this interesting forum.
One of the subjects was noting how she hates the snow and cold and warm and pleasant weather is what she and her family and friends are used to. So, maybe summer is just pleasant weather for traveling.