The challenge questions were pretty straight forward, the bonus however, was not able to get the right answer got a different day month combination where the frequency of lifts was 15.
I agree with @TonyA, @kmcdaniel and others who think the methodology suggested in the hint actually gave you an incorrect answer. Averaged over all the years, 15:00 on Saturdays in September is the hour where you're most likely to see the bridge lift. (with a possible slight adjustment for the number of Saturdays in September over the years as compared to the frequency of other days)