vCalendar 
An industry-standard specification, maintained by the IETF, that defines a format for exchanging calendaring and scheduling information. You can distribute vCalendars as e-mail attachments or make them available for downloading from a Web page. vCalendars hold information about event and to-do items that are normally used by personal information managers and group schedulers. Once the recipients have the vCalendar, they can drag and drop it into their vCalendar-compliant electronic organizers.