Walk around León on the night of December 7th, and you'll find the streets are packed with people of all ages, carrying white shopping sacks and happily shouting, "¿Quién causa tanta alegría?" to which perfect strangers shout back, "La concepción de María!" ("Who causes so much joy?" "The conception of Maria!")
December 8th is the holiday of the immaculate conception, called the Purísima in Nicaragua. On the night of the 7th, all throughout the country, Nicaraguans go to businesses and private homes in their town or city to "gritar" (yell). It's joyful yelling, of course, and an extremely joyful holiday.
It works like this: churches, businesses or homes that want to participate in the Gritería decorate with an altar to the Virgin Mary - sometimes they are elaborate, other times simple. They prepare baskets of whatever they want to give away (usually candy, but also sugar cane, whistles, headbands with feathers, combs, packets of instant coffee, ziploc bags of sugar, etc) and when the masses come around yelling or singing (there are several special "Gritería" songs), they hand out candy to whoever is yelling or singing.
The Gritería is probably the most "Nicaraguan" holiday/experience you'll find here, so don't miss it if you're in the country in December. You can find neighborhoods to "yell" in in any town, but León is the home of the Gritería and reportedly has the best turnout.