My wife and I are planning to visit Madrid for a week in the middle of May. I understand that is the time of the San Isidro festival.

What should I expect in terms of events, prices, crowds, etc., and are there any recommendations for things to do and see.

FYI - we are in our 50s, like all kinds of music, art, cultural stuff -- not into the club scene too much - love to walk and see historical sights.

Events take place in two locations. One, the Plaza Mayor; and two, the Plaza de las Vistillas, 15 minutes walk away near the Cathedral. The Plaza Mayor concerts tend to be earlier at around 10pm. Then everyone strolls down to Las Vistillas where the smell of fried tripe and sausage is thick in the air and the crowds lively and tipsy. The music is fun, mediocre stuff, which is endearing in a city as large as Madrid. Think Boney-M and Abba in broken English with overwhelming drum machines and shrill synthesisers, then a few traditional coplas to keep the old folks happy and some traditional Madrid songs to get grannies onto the floor and into the arms of swaggering teenagers with beer in plastic cups. Nobody seems to care that it's all a little tacky, and why should they? It's great fun. On the Sunday nearest the 15th there's a spectacular fireworks display over the Cathedral best viewed from the Vistillas park.
They're not the best festivals in Spain - you can't compare them to those of Sevilla. Remember, traditional "native" Madrileños are thin on the ground, so the fiestas are sort of swallowed up by the sheer size of the city. But you'll enjoy the goings-on all the same and if you're in town, you shouldn't miss a night or two. Be warned though, it always seems to rain during San Isidro. Or is that just my imagination?

Don't forget that San Isidrio is also THE time to see bullfights in Madrid too. There is one every day of the week, get tickets in advance if you can.
j.