Question: Will a rabbi agree to perform a wedding on Saturday before sundown?
Answer: Mazal tov on your daughter's engagement. You asked me about whether
a rabbi would officiate on a specific Saturday evening in the summer
Many Reform rabbis, including myself, will not perform a wedding on Shabbat -- that is, from Friday evening before sunset to Saturday evening following sunset. On the date you mentioned, sunset is at 8:40 p.m. in your location.
Jews do not marry on Shabbat because it is a time for rest. Anyone who has ever planned a wedding will tell you that it is not a very restful experience! Also, weddings in Jewish tradition are considered the forming of a binding agreement, which is antithetical to the nature of Shabbat. Shabbat is a time for communal joy, not for personal celebrations.
The date you mentioned also is the eve of Tisha B'Av, the darkest day of the Jewish year. This is the one day of the entire year set aside for mourning the state of the world's brokenness -- the day of the ancient destruction of the First and Second Temples. We recognize the brokenness of our world when we see war, poverty, violence and environmental destruction -- experiences that make us more distant from God. On all other days, we respond by trying to add to the world's joy; on this day, we mourn and fast. It is not an appropriate or propitious day for a wedding.
I regret that there may be a very few Reform rabbis who would choose to allow a wedding on that date. We are a movement that allows individuals broad discretion in the interpretation of Jewish tradition; such choices are the price paid by a movement that puts a high value on individual freedom. For myself, I would strongly urge you, your daughter, and her fiance to find a different date.
I also would advise you -- as I advise all families and individuals planning a wedding -- to find a rabbi to officiate before you set a date. You would not set the date for a surgery before finding a surgeon to perform it. A wedding is a major event in a couple's life and it deserves at least as much forethought when you seek a rabbi to officiate.