What happens at the Fiesta de la Almudena in Madrid on November 9?

I know November 9 is a holiday in Madrid for Fiesta de la Almudena. Does anyone know what happens on this day? This is what I found on a site: "The festival begins at 11 o clock in morning with a Mass that is held at the Plaza Mayor. A street procession follows thereafter at the Puerta de Sol, the Plaza Oriente and the Plaza Isabel II. Throughout the day at the Plaza de la Almudena flowers are proffered. The flowers bring hue and aroma to the streets. The festival gets over almost around 9 in the evening." Is it accurate? Do a lot of people participate?

The information you've got is right and yes it does draw quite a
crowd, mostly Madrilenos with the whole family in tow, including
buggies, so if you get in the middle it can become quite a
squash. The 'happening' bit is being there and participating!
The point of it is to honour La Virgin de la Almudena.

You already have most of the correct information yourself. The fiesta is called a "Verbena", and it usually lasts more likely until midnight or later, other than the flowers there is dancing in the streets, with people dressed as "chulapos and chulapas", which is the regional dress that dates back to the early nineteenth century. The people have "churros and chocolate" (crullers and hot chocolate) which is one of the typical things eaten in the verbenas. Everyone goes out to enjoy the atmosphere and the fiesta itself...
The main reason for the fiesta is to honor the patroness of Madrid La Virgen de La Almudena, the image of this virgin originally know as Santa Mar铆a la Mayor, has been venerated in Madrid since the year 1085 when the image was discoverd by king Alfonso VI.

You have the correct information but the party is quite corny and it takes place only around that square.