The shuttle goes on time about 40% of the time, truthfully. IT isn't the "never" that people seem to think. I read accounts here from a couple of people that they assumed it would never go June 8 so they didn't both to try. Sorry to hear that.
If the weather is good, your chances are not bad. Technical problems do come up but overall it's not as common as you might think. Yes, 117 June 8th was the first on-time launch since the Columbia tragedy. All four post-Columbia flights had at least one scrub and went the second day, and in one case the third day. But finally we had another on time launch. Maybe it's the start of another string...
There are years when they had several in a row all go on time. If you take, for example the period from September 2000 to July 2001, there were seven shuttle missions and six of them went right on time on the first try. And the eighth mission in August 2001 had to wait 24 hours and the thunderstorms stayed at bay on the second attempt.
The main threat in August at 7pm is Thunderstorms. Truth be told, the same was the case June 8 and we had a beautiful day (rain in the early afternoon cleared out and it was crystal clear by launch time).
Storms in Florida usually form daily in the summer, and depending on mid and upper level winds they may form in the middle of the state and move to the coast; form in the middle and move west; form at the coast and move inland. By 7pm they have usually 'rained' themselves out or moved away from the area, but sometimes you may have leftover anvil clouds that don't dissipate until the sun goes down.
The truth is, it's 50-50 and there is no reason to assume it's not going. Don't believe those that say that. There are 15 shuttle missions left at the most and it is well worth making the effort to see one!