Final answer:
The plays open during the Dionysia, a spring festival held in ancient Athens to honor Dionysus, which featured dramatic competitions with tragedies and comedies by playwrights like Sophocles and Euripides.
Step-by-step explanation:
The holiday being celebrated when the plays open is the Dionysia. Specifically, the City Dionysia, or Great Dionysia, was an important festival held in ancient Athens in the spring to honor the god Dionysus. It included dramatic competitions where poets presented their tragedies and comedies, and the winners were awarded prizes, much like the modern-day award ceremonies. This significant cultural event fostered the development of theater as a form of art and entertainment, with contributions from well-known playwrights such as Sophocles, Euripides, and Aeschylus.