Well, you can start by imagining one from your experience.
Talk about what was the role you played during the sports day. you could be a participant (talk about how long u've trained for this day, and you are excited). a friend of yours is volunteering as first aid officer on stand by.
like any other sports day, there'll be a marching performance from 'houses/teams/ clubs'. say what you feel at that time, amazed, excited/impressed by the marching performance, etc/