An event is historical when
1. such an event had not taken place before.
2. such an event has a wide impact on a huge number of people, or over a large geographical region.
3. such an event does not occur frequently. Like visit of President of America or Russia to Nepal for example. Like another planet coming close to Earth and being visible very clearly. Like solar eclipse of a type that happens once in a century.
4. Such an event has toppled all expectations and reversed the guesses of many experts.
5. Such an event changes the course of action or future of people. Like Independence of a country or formation of a state.
A historic event can be world-wide, country-wide, state-wide or city-wide even.