This is an excellent question. As you can imagine, there will be some difference of opinion. However, there are three main opinions.
First, some scholars say that Luke 12 refers to the destruction of the temple, which took place in 70 A.D. under the Roman emperor Titus. Most scholars take this view. You can read about the account of the destruction of the temple in Josephus, the Jewish historian.
Second, other scholars say that these events refer to an future time period, where there will be a cosmic upheaval. These scholars, of course, take a more supernatural view of history. In other words, Luke is predicting what will take place at the "end times."
Finally, still other scholars say that what is being stated in Luke 21 is spiritual lessons of what has taken place in history and what will take place. In other words, Luke 21 lays out a cycle of hardships that take place during history.