In the 7-1/2 years that I've run editorial sites about European travel, I've noticed that traffic has usually (but not always) peaked in late May or early June. The same thing happened this year, with traffic dropping off a bit in mid-June. Some of my affiliate programs' bookings have also slowed down a little, although hotels have remained strong.
I ascribe the traffic drop to: (1) College and univirsity students going home (and, in some cases, heading for European vacation); (2) A decline in visits by armchair travelers, schoolchildren, and other casual visitors.
As for bookings, I think there may be two peak times: right before summer begins, and again before the busy vacation month of August.
Now that August is approaching, I'm seeing a new surge in both traffic and affiliate bookings.
BTW, my slowest time of year is usually late November and early December, with both traffic and bookings picking up sharply after New Year's.
Side note: I keep an eye on my Alexa traffic rankings and those of other editorial travel sites, and I've been edging up a bit in comparison with sites like Travel and Leisure. That would tend to support my belief that, for editorial travel sites in general, the peak period is immediately before--not during--the summer vacation season.