The Emperor of Japan’s birthday, which occurs every December 23rd is an occasion filled with pomp and circumstance and is carefully choreographed. The royal family, weather permitting, makes several appearances on the balcony in the morning. Other celebration activities take place inside the palace during the afternoon.
Upon exiting the palace, visitors may walk through the East Gardens of the Imperial Palace but general access to the gardens is closed for the Emperor’s birthday. I’m guessing the gardens are closed because security personnel are needed at the palace itself but I don’t really know.