Why July?
I mean most of the world (except us) takes August off.
Does he want to beat the rush?
Well, maybe that is a little charity for the people who are at the place where he would be visiting...again if he went on a vacation. People who would normally be on vacation in August might need to cancel their plans if the Holy Father was to visit...in July, his vacation would cause far less impact to an area than during August.
Having said that, though, as long as I can remember, Popes have always been away from Rome in July and August. Other than the time that they would be on "vacation" (if one is taken at all), they spent the remainder of the time in Castel Gandolfo.
(Of course, there are notable exceptions, like during WWII and the time from the establishment of the Italian Republic and the signing of the Lateran Treaty, but those are exceptions)