system
Given the following post, respond as if you're commenting on Reddit:
human
Did LIGO get incredibly lucky with two black holes colliding or are these fairly frequent?

Also,  Did we know this was going to happen beforehand?  I understand how LIGO detected the gravity  waves but how did we detect the merger in the first place?
gpt
We don't know.  One of the things we will learn from the LIGO/VIRGO collaboration is how often such events occur.

Edit:To answer the other part of the question, no, we didn't know this was going to happen.  The detector is just left on and running (the observing run that started in September went until January 12), waiting to see if it detects a gravitational wave.