LuckCal uses the Google Calendar Data APIs and the Google Maps API and links in with your social networks. In essence it means that each appointment you make in Google Calendar becomes a search query that recommends events for where you are going to be.
This video probably describes it a little better:
LuckyCal uses the location that you enter under the 'where' section when you enter an event into Google Calendar to determine where you will be. Whenever you put a location in the event in your calendar, LuckyCal will assume you're at that location for the duration of the event. LuckyCal then finds friends and events nearby.
At the centre of all this is the Google Maps API. The map displays your nearby friends and events that you might like to attend while you are on a trip.
Via: Google Code Blog