The current calculation is it assigns a random integer from 222 to 2013 inclusive to variable 3921 and then checks to see if it's less than variable 44. Variable 44 is assigned a random integer from 0 to 255 every time you go to sleep. Taking all this into account, I have no idea what the actual probability of triggering the event really is, but I suspect it's somewhere close to once in a fucking ice age or something.
Before 0.100a, there were some additional operations that ostensibly made it easier to trigger, but they're gone now.