I prefer to do it for 10 minutes every day, rather than one day every year.
YES! We must do routine activity to conduct environment education or sustainable development education for our kids. 10 minutes every day even better than 1 hours of lesson every week.
After all, with just 5 minutes for a simple game and another 5 minutes for discussion you can pack this simple activity, integrate it with your biology, math, physic, economy, social, language lesson, etc.
See? A piece for everyday is better!