There are so many things to say about the tragic story of Romeo and Juliet that its creation sometimes gets short shrift.

Unfortunately, scholars cannot pinpoint the exact date that Romeo and Juliet was written, but they have narrowed it down to some time after 1593 and before 1596. It was first printed in 1597, in quarto format.
William Shakespeare was born in April of 1564, in the English town of Stratford-upon-Avon. His family was modestly affluent when he was born, and his father was prominent in local government, although that changed by 1576 his family began to suffer a reversal of fortune.
When he was 18, he married Anne Hathaway, and together they had three children. By the age of 26, he was writing and producing plays. Around 1593, an outbreak of plague in London caused theaters to be shuttered, and Shakespeare spent the time writing poetry.
In 1594, he was once again writing plays and acting, and he eventually became a partner in the group that built the Globe Theater on the southern bank of the river Thames. Shakespeare retired to his hometown around 1613, and died in 1616.
Of Shakespeare's works, 38 plays, four narrative poems, and 154 sonnets are extant. Six of the plays are thought to be collaborative works with other authors. There are also several plays that were at one point in history attributed to Shakespeare, but which are no longer considered to be authentic.
Romeo and Juliet was written primarily in blank verse in iambic pentameter; that is, it has a regular meter consisting of five iambs (an unstressed syllable followed by a stresses syllable) per line, but does not rhyme. About one-sixth of the play rhymes, and one-seventh is written in prose. The different styles are mixed together throughout the play.
The play tells the story of "a pair of star-cross'd lovers" in Verona: Romeo, the scion of the house of Montague, and Juliet, the young daughter of the house of Capulet. The two families have a long-standing feud.
Romeo crashes a party at the Capulet’s' house and the two fall in love. With the help of Juliet's nurse and a friendly friar, they conspire to marry secretly. Romeo kills Juliet's cousin Tybalt in a street fight and is banished from Verona; Romeo and Juliet are both distraught. Juliet's father decides to wed her to a nobleman in a few days to take her mind off her grief, not knowing she is already married to Romeo.
Juliet seeks refuge with Friar Laurence, who offers her a potion that will make her seem dead for two days, long enough to be interred in the family crypt, where Romeo can then meet her. The friar writes a letter to inform Romeo of the plan, but it fails to reach him, and he rushes to the crypt with his own vial of poison. After seeing Juliet's still body, he drinks the poison. Shortly thereafter she awakens, finds Romeo dead, and kills herself with his dagger. In their joint grief, the two families reconcile.
During the more than four hundred years since Shakespeare wrote Romeo and Juliet, the play has been produced hundreds, if not thousands, of times on various stages big and small. Especially in the early days, directors felt free to take extraordinary license with the work, sometimes even allowing Romeo and Juliet to live happily ever after.
The story has not remained a play, either; it has been adapted for opera, symphony, ballet, movies, and musical theater. Two of the most famous movie versions are Franco Zeffirelli's 1968 version, which was the first to use a young cast, and Baz Luhrmann's 1996 Romeo + Juliet, which retains much of Shakespeare's language but updates the setting and characters.
In the modern era, the story of Romeo and Juliet has also been transformed into a contemporary tragedy, most famously in Leonard Bernstein's West Side Story. Even more recently, High School Musical borrowed heavily from Romeo and Juliet, proving that the answer to the question, "When was Romeo and Juliet written?" is: in every generation it is written anew. Shakespeare's genius is timeless and infinitely adaptable.