The xkcd strip of the day features the concept of geohashing. It works by taking the square of your lat/lon coordinates (lat, lon, lat+1, lon+1) and then deciding the decimal part by hashing a date concatenated with the opening at the DOW index. The calculated location is the meeting point for that day. As the calculation of the points depend on the DOW index for that particular day, it’s impossible to guess the location the day before the meeting is happening. Read more on the xkcd wiki!
Anyone in Fredrikstad/Sarpsborg/Halden up for this?