Going back over the last 20 years, here's the list of all 27 days in July that have featured a high of 75 or cooler. Notice that the vast majority of those include at least 0.01" of rain...and that there are only six days in the last 20 years where it was not only 75 or cooler...but where it didn't rain more than a trace.
Yesterday was one of those six days...and the last time we've been here, did that was July 2nd, 2007 when the high was 75 degrees.
While technically we picked up a trace of sprinkle activity at the Airport, for the sake of argument since we didn't pick up anything beyond that, it's considered a "dry day" since precipitation wasn't measured beyond a trace.
What's probably a bit more unique about yesterday is that we also were a notch cooler than our average LOW month-to-date.
Through yesterday, our average low in Philadelphia is 74.52 degrees this month. Given today's daily low will come in below 74.52, that average will drop a couple of tenths more today (probably down to around 74.2 or so).
The last time we've experienced a day with a high temperature below the monthly average low was July 8th, 2005...the average low for the month was 70.2 and the high on July 8th was 69 degrees thanks to the remnants of Hurricane Cindy working through the region with around two inches of rain. It doesn't happen often...and given our trend for somewhat cooler than 74 for each night through the end of the month, it probably won't happen again this year. However, yesterday's cool high is a bit unique given the general lack of rain and given how hot it's been. Hopefully you got to enjoy it!