We are traveling to Madrid in May and although unplanned, it appears that we will be there during the May 15 Fiesta de San Isidro. Is this a day-long event, or over a period of several days? What should we expect...crowds, banks closed, etc?

It takes several days, but except the 15 everything will be opened as normal.
Crowds ? no, not more than usual in Madrid, except around Plaza Mayor where they held concerts in the evening during the festival.

Info on Fiesta de San Isidro:
http://www.gomadrid.com/fiestas/san-isidro.html

Muchas gracias Kenderina y Revulgo!!

We were there last year at that time. It was very nice. There were a few special things/events but by no means huge crowds or big exibitions that blocked traffic.
In Plaza Mayor they had several couples in the traditional outfits that are no doubt mentioned in the links provided by the others, I liked that kind of thing.

I was told by one local that they have cut down alot on the free dance shows & concerts at Plaza Mayor. But in general I think it will enhance the visit.

ams

We have enjoyed the several times we have been in Madrid around San Isidro. As mentioned above, it adds a little festivity (little boys and girls dressed up) and some nice events, but it's not big enough to create any travel problems. I was thinking that even Corte Ingles was closed (?) or maybe had holiday hours on the 15th, but other than that kind of thing, no hassles.

Enjoy! May is a great time to be in Spain!