... "the end of summer". Since we are still a tad over a week away from the beginning of summer and the end of summer isn't actually until late September, I would guess you're alittle premature trying to schedule already. Therefore, I would just wait quite awhile before trying again. I've never even tried to schedule more then 3 weeks in advance although I know some guys do. So many things can change between now and then.
If she will actually be coming to your area soon, I would wait about 4 days (more or less) from my first attempt to contact her and then try again. If I still didn't hear from her at that point, I would just move on.