JOI Open Contest 2020 will be held from Sunday, September 6, 04:00 UTC to Monday, September 7, 04:00 UTC. You can start at any time and the duration is basically 5 hours. Note that the duration becomes shorter if you start after Sunday, September 6, 23:00 UTC, i.e. less than 5 hours before the contest ends.
There will be 3 tasks. Each submitted source program must be written only in C++. Problem statements will be provided both in Japanese and in English.
You can find the details in the contest page.
Past contests are also available in this page. Since the judge server isn't available now, please download testcases when you want to test your code.
Good luck and have fun!