Evergreen plants, like Holly, Ivy and Mistletoe have been used for thousands of years to decorate and brighten up buildings during the long dark winter. They also reminded people that spring would come and that winter wouldn't last forever!
The Romans would exchange evergreen branches during January as a sign of good luck. The ancient Egyptians used to bring palm branches into their houses during the mid winter festivals.
In many parts of Europe during the middle ages, Paradise plays were performed, often on Christmas Eve. They told Bible stories to people who couldn't read. The 'Paradise Tree' in the garden of eden in the play was normally a pine tree with red apples tied to it.
Now the most common use of green at Christmas are Christmas Trees.