Many countries celebrate the first of May as 'May Day', although most people no longer have any idea what the first of May actually signifies. As with nearly all current holidays and customs, May Day was an ancient celebration in the Wheel of the Year, called (and still called such by many practitioners) 'Beltane' (or Beltaine), which means the fire of Bel, who is the 'bright or shining one'.
On the eve of May first, the ancient Celts lit fires, using nine of the sacred woods, and herd animals (goats, sheep, cows, etc.) were ritually driven around such fires or between a pair of fires, to purify and protect them from disease and harm, as well as to ensure fertility. Couples might decide to join together at this time of year in a 'hand fasting' ceremony and likewise walk together round a fire or jump over it, to seek protection, success and fertility in their joining.
May is a month of sensuality and sexual revitalization. A well known May Day celebration which originated in ancient Germany--and is still practiced in some areas--is circling the May Pole. Few now realize that the Pole itself is an age old phallic symbol and the act of dancing around it is an ancient fertility rite that goes back thousands of years. The entire celebration of 'May Day' is to welcome the rebirth of Spring and revitalize sexual energies that have been put on 'hold' through the long, cold winter. (No wonder most religions and many countries have 'tamed down' May Day celebrations.) Spring flowers abound in May and 'rebirth' is visible all around us, as trees bring forth new growth and animals begin their mating rituals that will produce offspring in Summer.
Green is the color of Beltane, to symbolize the rebirth of new growth, as well as to honor the 'Green People', such as Pan, who have pointy ears and little horns. Some even consider the legend of Robin Hood, who was decked out in green from head to toe, might symbolize Spring itself, as he gave forth unto the people new life and hope, while his companion, Maid Marion, represented Mother Nature.
Green symbolizes the return of 'life' that was stolen from us for many long months, by Winter. Likewise, the emerald, a deep, dark green gem, is the birthstone of May. This month is a time of fertility and new beginnings, when we should all give thanks to the Earth and to Mother Nature for our bountiful blessings of light and love, brought forth by Spring.