For the Congress Avenue Bridge bats, the scariest time of year is 4th of July, when the annual fireworks display takes place just a few hundred yards to the east of the bridge.  The bats are emerging at dark this time of year, exactly at the time the fireworks start.  The emergence peaks around 9:30 pm this time of year, just when the fireworks are reaching their crescendo, seen in yellow on the left.
We normally see greatly reduced emergences for a number of days after the 4th.