Summer workers/vacation temps. Lots of buisnesses run on skeleton crews, stores having adjusted closing times, public transportation runs on adjusted schedules (less busses etc), construction is mostly closed down. The only buisness that stays open and runs well is hotels/restaurants/bars etc. The rest almost grinds to a halt. Hell, my local docters office has adjusted visiting hours, as half of the docters are on vacation, while the ones staying behind wait for their tiurn later in summer.
Europeans go away during vacations, that is definitly a cultural difference. It is seen as a right of every employee, thank you socialist democrats :). Hell, they even get paid WHILE being on vacation. It's called vacation days, and everybody gets an allotment per year, usuable as they see fit. Most use them during the summer months for a nice 2-3 week trip. And yes, winter is a season better suited for naval action.