訳:It is a historical date.
I apologize that my answer is in English, but I am not confident enough to answer in Japanese.
There is nuance between the words "historic" and "historical". The word "historical" simply refers to something from the past. The word "historic" refers to something from the past that is also important.
This means that a "historical date" is one that occurred in the past, but a "historic date" is one that occurred in the past and is important to history.
Here is a link with a brief explanation: https://en.oxforddictionaries.com/usage/historic-or-historical