We use estar in order to indicate a place where a known object o person is located:
El libro está encima de la mesa – The (known) book is on the table
Juan y María no están en casa – Juan and Maria are not at home(we know them both)
We use ser when a known event occurs:
El concierto es en la Plaza Mayor – The (known) concert is on the Main Square
Los exámenes son en junio – Exams (that we know) are in June
When speaking about unknown objects or events, we must use hay. The English equivalent of hay is there is or there are. Note the word order: in Spanish we say first the known information, in this case, location:
Encima de la mesa hay un libro – There is a (unknown) book on the table
En Madrid hay muchos turistas – There are many (unknown) turists in Madrid