The event is probably the start of (or early phases of) the construction of Tama New Town. In real history, large areas of nature were exploited in order to build residential areas, due to the rapid urbanisation of Japan during this time. This would set the pom poko calendar as starting sometime in the mid 1960s. Since the movie focused on a particular group of Tanukis, it can be narrowed down to be the year when this group of Tanukis was forced from their homes due to human expansion.
The movie then starts 31 years after pom poko. Possibly this would be around the year 1994 when the movie was released. Making the pom poko calendar starting event around 1963-ish.