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Health and Sports Day  also known as Health-Sports Day or Sports Day,

is a national holiday in Japan  held annually on the second Monday in October.

As It is a day to promote sports and physical and mental health, many schools and businesses choose this day

to hold their annual Field Day ( Undō-kai), or sports day.

In 2020, Health and Sports Day will be permanently renamed Sports Day (Supōtsu no hi).

(Wikipedia)

I’m going to play tennis on this day.