The first Arbor Day in the US was held in 1872. Since then, the event has really taken root. On the last Friday of April, people across the country mark the occasion by planting trees, often dedicating them to a loved one. It"s also a great day to celebrate trees themselves. As well as providing homes for countless species of animals, trees absorb carbon dioxide and clean the air we breathe. They also help to keep cities cooler and can even play a role in preventing floods.