Explanation:

- World Water Day is observed on 22nd March each year. In 2019, the theme was "Leaving no one behind," highlighting the importance of ensuring access to clean water for marginalized groups. This aligns with the UN's Sustainable Development Goal 6, which aims to ensure availability and sustainable management of water for all.

- World Forest Day, also known as the International Day of Forests, is celebrated on March 21st. It focuses on raising awareness about the significance of forests and the role they play in conserving biodiversity.

- World Environment Day is celebrated on June 5th. It aims to encourage worldwide awareness and action to protect the environment, focusing on various themes each year.

- World Petroleum Day doesn’t have a specific annual observance day worldwide. It's typically marked by industry events focusing on the importance of petroleum in global energy.