depends which part(s) of Europe you plan to go

Can be very different weather even during the same period.

Jul/Aug lots and lots of tourists in most places. Very hot in Spain and Italy, especially the southern parts. I felt like a dog when we were in Rome during late July and almost got sick at Pompeii. Finally got sick in Spain a week later, and got worse and worse by the time we returned to Paris. Had to rush back to London for medical treatment.

Also our trip was a driving / camping trip (Camping in Europe is completely different from U.S. - it is not wilderness - all our campsites are either in the centers of big cities or at the edge of the city - a cheap way to travel because we were very tight on our budget back then. That might also contribute to the fatique that led to illness.