I would strongly suggest July over September, for my personal preferences weather and light-wise, but you may want to check annual temp and daylight norms for those months yourself. We were there in July, and I loved the long sunlight hours (it did get dark plenty of hours in the evening, but there was a lingering dusk).
DBF celebrated a birthday during the cruise, which we had noted when booking. The registration forms asked if any event was being celebrated, we noted the date and event, and the ship supplied a bouquet of balloons outside our door that morning, and a birthday cake with candles at dinner. Particularly as you plan to celebrate your fifth year anniversary, I imagine you can so specify and then provide the date you wish to observe it, even if the actual date is outside your cruise schedule.
And, to correct an earlier entry, our Alaska cruise was on Princess, we were on Royal Caribbean for a different cruise. I enjoyed both lines, but clearly have a bad memory for the cruise line vs location! One thing I do remember is that we booked our cruise ourselves, when DBF learned of a sale on Princess bookings in the December before our July cruise. A couple who we got to know during the cruise had used an agent, and they were in the process of booking a different line (same itinerary) when their agent suggested booking the one we were on during the same sale. As you are planning well in advance, just thought I would add the suggestion to sign up for emails on discounts available at some times of the year. The cabins booked after the sale were a good bit more, and we were also able to get a choice of cabin location from a broader array by booking more than half a year ahead.