Just a few thoughts:
Brian's line of thinking leads me to a different conclusion. If there is ample time (and there is) to do both, then why change it?
It would likely not be very easy to find another date that works later in the semester. The later it gets in the year, the busier faculty get trying to wrap up the semester. Also, although unlikely and unpredictable, the weather could be uglier for a cookout later in the year.
Susan has access to most faculty and department calendars and attends both faculty and staff meetings in the department. It's safe to assume she chose this date after consulting those because it includes most of the department. That said, when you email her, maybe include a short, friendly note encouraging her to consult the GSA calendar and events before scheduling departmental events so that there is no overlap.
Hope that helps!
Joey