Step 1: Understanding the history of the Olympic Games.
The ancient Olympic Games were a series of athletic competitions held in honor of the Greek god Zeus. These games were first held in Olympia, Greece, and they became a major event in ancient Greek culture.
Step 2: Identifying the year.
The ancient Olympic Games began in 776 BCE, and they were held every four years until they were abolished in 393 CE by the Roman Emperor Theodosius I.
Step 3: Conclusion.
Therefore, the correct answer to fill in the blank is 776 BCE.