Respect for the Aged Day (Keriou no hi) is a national holiday which celebrates annually to honor elderly citizens. We usually give flowers or something special to our grandparents meaning of thanks and respect.
On this day you might get something special if you are with your grandparents, such as free entrance fee at the amusement parks or drink services at some restaurants and so on. There are also many events held to communicate and interact with elderly citizens.
At the local kindergartens and elementary schools where I live, we invite the elderly or visit their nursing home as an event of Respect for the Aged Day.