There are many Buddhas. Buddha can mean the historical Buddha, who was born Prince Siddhartha Gotama in the foothills of the Himalayas over 2,500 years ago. Or Buddha can mean one who has achieved enlightenment. In this article, I'll give some information about the historical Buddha of this age: Shakyamuni Buddha.
This account begins:
Shakyamuni Buddha, the historical founder of Buddhism, was born in India 3,000 years ago. There are various opinions concerning the exact dates of his birth and death, but according to Buddhist tradition, he is said to have been born April 8, 1029 BC and died on February 15, 949 BC, although other Buddhist scholars place his birth five hundred years later. No definite conclusion has been reached.
Shakyamuni Buddha was the son of Shuddhodana, the king of the Shakyas, a small tribe whose kingdom was located in the foothills of the Himalayas south of what is now central Nepal fifteen miles from Kapilavastu. Shakya of Shakyamuni is taken from the name of this tribe and muni means sage or saint. His family name was Gautama (Best Cow) and his given name was Siddhartha (Goal Achieved) though some scholars say this is a title bestowed on him by later Buddhists in honor of the enlightenment he attained.
So it is evident that many Buddhists observe Buddha's historical birth on April 8, even though the exact date remains in question. Modern archeological and historical research do confirm that Prince Siddartha Gotama lived around this time.
Pilgrimages are made to Lumbini garden, reputedly the site of his birth. An early pilgrim was King Asoka who had much to do with proliferating Buddhism.
Historical Places of the Buddha says of his birthplace:
Lumbini: This is the sacred place where the Buddha was born. It has been identified with the site of Rummindei in Nepal. Here there is an ancient shrine with an image representing his birth as Prince Siddhattha. At the site a pillar remains which is engraved with an inscription commemorating the Emperor Asoka's pilgrimage there in the twentieth year after his coronation. There are ruins of a number of monasteries from the time of Asoka.
Confusing to the Buddhist scholar are the many different terms for Buddha, and so many classical depictions of Buddha, each to represent different aspects of Buddhahood. For one thing, terminology changes as Buddhism spreads from India to Tibet and thence across northern China and down to southeast Asia, then to Japan and Korea.
There are Sanskrit terms used alongside the terms in each native country. For example, the same figure, the embodiment of compassion, is Avalokitshvara in Sanskrit, Chenrezig in Tibetan, and Kwanon in Japanese.
BUDDHA (Skt. = Tib. sangyé). Awakened one; person who has achieved enlightenment (Skt. bodhi, Tib. changch'ub). For the Mahayana, the concept of Buddhahood is extended from the historical Buddha Shakyamuni, he is seen as an emanation of a Buddha-nature (dharmakaya) underlying all phenomena. See TRIKAYA DOCTRINE. Within the Mahayana, there are numerous Buddha-forms, such as Amitabha, Akshobhya and Vairocana, and some of them can be encountered within meditation.
Additionally (and this is by no means a comprehensive list) there is Medicine Buddha; female representations such as Red, Green, or White Tara, and Maitreya Buddha who is the future Buddha to come.
A study of Tibetan thangka paintings is helpful in sorting out many of these. The lush Thangka Gallery at www.thangkapaintings.com gives some idea of these various representations. Every figure in a traditional thangka represents an aspect of the teachings, and every symbol has a meaning as well. Another attractive thangka site is www.thangkapaintings.com Mystic Buddha. One of the many traditional thangkas depicts the whole life of Buddha; an example can be seen at this source.
Observances of the birthday vary from lineage to lineage and from country to country. There is a legend that, at the time of his birth in Lumbini garden, the heavenly gods celebrated by creating a rain of amrta which I take to be a heavenly nectar. Today this is symbolized in some temples by pouring sweet tea over a statue of the infant Buddha.
Finally, should you wish to read a book on the subject, pick up Old Path White Clouds by Thich Nhat Hanh. The book is not in chronological order, so some digging is necessary to use it as a reference, but it is richly detailed and lyrical. Here is the section on historical Buddha's birth: Mahamaya, Siddhartha's mother, had a premonitory dream before giving birth to him:
A magnificent white elephant with six tusks descended from the heavens surrounded by a chorus of beatific praises. The elephant approached her, its skin white as mountain snow. It held a brilliant pink lotus flower in its trunk, and placed the flower within the queen's body. Then the elephant, too, entered her effortlessly, and all at once she was filled with deep ease and joy.
\The king summoned all the local holy men to divine the meaning of this dream. Their conclusion: "Your majesty, the queen will give birth to a son who will be a great leader. He is destined to become either a mighty emperor who rules throughout the four directions, or a great Teacher who will show the Way of Truth to all beings in Heaven and Earth."
...It was the custom in those days for a woman to return to her parents' home to give birth there. Mahamaya ...set out for Ramagama, the capital of Koliya. Along the way she stopped to rest in the garden of Lumbini. The forest there was filled with flowers and singing birds. Peacocks fanned their splendid tails in the morning light. Admiring an ashok tree in full bloom, the queen walked toward it, when suddenly feeling unsteady, she grabbed a branch of the ashok tree to support her. Just a moment later, still holding the branch, Queen Mahamaya gave birth to a radiant son.
Fortunately the queen had attendants with her to wrap the child in silks, and to escort mother and son back to the palace.
Mahayana Buddhism teaches that there were many Buddhas before this historical Buddha, and there will be many--in fact thousands--after this one. In some dark ages, there is no Buddha to guide humanity. So we are most fortunate to be born in a time when there is a recognized Buddha, and the teachings are so widely available.