Help, Can anyone give me advice on whether it would be a bad idea to do our visit to Koyasan on June 15, when they hold an annual festival? Would it be a bad day to visit because of too many people, or a good day because of the festival activities? Thanks.

We went to Koyasan on Aoba Matsuri a number of years ago, and it was really worthwhile. A parade with floats, special performances, and lots of people. You'll see pilgrims (ohenrosan) honouring Kukai dressed in white with staffs and hat.
You should immediately get your accommodation, though, as it's just two weeks away. A day trip would really limit your time at Koyasan, and the trains could be very crowded. It's one of our favourite places in Japan, and doesn't get a lot of tourists in comparison with other spots.

Thanks WillJame. We are going to go for it, even though its on a Sunday this year - which may make it more crowded.

You'll enjoy it. Try to get out to Okonuin, where Kukai is enshrined, while you're there. The surrounding tall forest and cemetery is most impressive.
Except for crowds along the parade route, the town itself has lots of room to move around and explore.

We spent 3 days i Koyasan in April this year and it was one of the highlights of our round the world trip. I don't know of this festival but Koyasan was one of the highlights of our trips so far. Why was that? Maybe it was the peace and serenity of the place that made it special. For that reason, I would be inclined to visit when it is not so busy.

Walk through Okunoin around dusk.
Ethereal. Probably the highlight of my two weeks in Japan a couple years back.

I've posted this on another forum, but if you were interested in comparing a Buddhist matsuri with a Shinto one, there's the Lily festival at Nara:
Just browsing through Kansai Time Out and see that the Saikusa Matsuri (Lily Festival) at Isagawa Jinja is scheduled for June 16th. It's one of the best things I've ever witnessed if you're interested in the rituals at Shinto shrines and in seeing dancing by the shrine maidens. Right up there with more famous Nara events such as Omizutori or Shichi-go-san. If it fits in with your plans, it's well worth a trip. You'll have to locate this little shrine on a map--it's an easy walk from the centre of town, but a little out of the way. And best to verify the date. Another online source has it on the 17th.

Just Returned from Japan yesterday and we did attend the Aoba Matsuri on June 15 (Sunday). Crowds were not a problem at all. Viewing the 'parade' was worth scheduling to go to Koyasan on that day. During the past 2 weeks (June 9 -21) I found fewer tourists than I expected, either Japanese or Foreigners, everywhere we went in Japan.