Today is 建国記念日 (Kenkoku-kinenbi). In English, “Anniversary of Japan’s Foundation” or “National Foundation Day”.
In theory, people are meant to reflect on their love of country and what it means to be “Japanese”.
In reality though, hardly anyone does anything special on this day. Many generations ago, it may have been different…but today, it’s just seen as “a day off” to most people.
You can see Japanese flag hung from street lights and also on the front grill of city buses…but that’s not particular to this holiday—that’s done on every national holiday in Japan.