How do you purchase tickets for the evening concert at St Chapelle. Can you buy them the "day of" onsite or is it better to buy online. I'll be there in mid to late May but would like to choose a day when the sun will be streaming through the windows rather than guess on a date. Any suggestions ? Thanks ......

I bought my tickets online. My purchase was based on performers. Luckily we'll have a dinner reservation afterwards.
I purchased from classictic.com

On a day in May in 2011, I bought "day of" tickets without difficulty -- but of course if you do that, you need to be prepared for a sold out concert.

I bought mine at the location a day or so before the concert.

Went to a concert there the night before Easter 2012. Got there early and bought them at the line to get in. Was not a problem then, but of course it is better to buy them as soon as you make the decision to go. They can always add a few more chairs but the venue is not that big. Attending a concert there is a wonderful experience. Beautiful music in a beautiful setting.

I too bought mine at St Chapelle

Yes, you "can" buy day of, and concert time IF and IF it isn't sold out (as others noted). We were really disappointed but it was All Souls Eve and a really special choral concert.