To describe what a sparkle shower is, let's start with flash mobs - these are events where lots of strangers visit a location together, perform an action (usually something unusual or surreal done for its own sake, though sometimes with political or commercial intent).
The participants in flash mobs
get to enjoy being part of the mob, and any embarrassment they feel about their weird behaviour is diluted due to the size of the group.
People observing flash mobs are often surprised and disconcerted at first. These days, enough people have heard of flash mobs that they are unlikely to be confused for long, and will probably be left smiling in amusement.
Sparkle showers are similar to flash mobs, but different. They're similar in that sparkle showers are events where the participants perform a surreal action, but different because sparkle showers don't require the participants to meet up. Sparkle showers might be organised so that all participants act simultaneously, or be spread over a period of time. They might involve participants in various locations, or all in one city. (Flash mobs themselves could be described as a subset of sparkle showers where the time and location are highly focused.)
Participants in sparkle showers, like participants in flash mobs, get to feel the sense of belonging that comes from being part of something bigger than themselves. However, they also have to be prepared to behave bizarrely in public (to 'sparkle') without the comfort of having others doing the same right beside them. As individuals, they will stand out more, which arguably takes more courage.
As a group and an event, however, sparkle showers are invisible at first. People who see an individual sparkle will probably be initially amused. They might assume the person they saw is mad; they will certainly assume the person they saw is acting as an individual.
It will only be later, when they see someone else performing the same action, or when a friend mentions seeing someone else doing it, that they will become disconcerted, and start wondering how and why these different people are connected. Depending on the action observed, this might create different emotions (seeing several people leave potted flowers on the pavement might leave one feeling strangely optimistic; seeing several people stumbling around zombie-like might leave one feeling rather uneasy :-) ).
As events, sparkle showers can be tweaked in more ways than flash mobs. For example, the number of participants could grow over several days, leaving the public wondering whether a new fad is forming, and then surprised when suddenly they no longer see anyone doing it. Whereas one way to measure the success of a flash mob is the level of media coverage it receives, some sparkle showers could be ongoing (e.g. one action per participant per week) to see how long they can bewilder the public while remaining under the media's radar.
There are some other side-effects of interest, too. For example, a participant turning up to a flash mob would feel disappointed if he or she finds no-one else turned up. Participants in sparkle showers, on the other hand, will expect to be alone for their action, but have a slight chance of being joyfully surprised to see another participant.
(This also opens up the possibility of fake sparkle showers - getting a friend to do something bizarre, not knowing that they're the only person in the world doing it.. ;-) )
(Finally, maybe people would become more accepting of unusual behaviours and people, since they can't be sure whether someone behaving unusually is really a just random lunatic, or actually the first sparkle in a new sparkle shower.)
As for the name, I was imagining seeing these events marked on a map over time. Flash mobs would be bright, sudden flashes on one spot, while sparkle showers would glitter and sparkle like city lights at night. :-)