When people receive notices or invitations in the mail, there is no mystery whatsoever about the information. Regardless of the organization hosting a gathering of people, recipients will understand how such a notice arrived at their door. In the middle of the day, while they were at work, a postal worker came and deposited it in their mailbox.
Mail is increasingly a commercial resource as the majority of it is quickly becoming solicitations, bills and catalogs. Advertising an event that is meant to be interesting, mysterious or exciting is simply not quite as effective when done through the postal service. As a result, a slightly different tactic might be necessary.
To create an aura of mystique around a gathering of some kind, organizations should consider using door hangers. These printed materials need to be placed on doors by hand, but the experience of waking up to one that has mysteriously appeared on one's house or at an apartment is something people don't encounter every day. The simple fact that such a thing has been placed at one's own home means that an actual person came by. This lends messages a bit more weight and makes it so that people will pay just a bit more attention to the information or invitation that they contain.