Which ancient Greek festival honoring Apollo was held at Delphi every four years?

The ancient Greek festival honoring Apollo and held at Delphi every four years was the Pythian Games.

The games were part of the major Panhellenic festivals and were staged at the sanctuary of Apollo at Delphi. Athletic competitions formed an important part of the program, but the festival also included musical and poetic contests, reflecting Apollo’s associations with music, prophecy and the arts.

The Pythian Games were traditionally connected with Apollo’s victory over the serpent Python, whose name supplied the festival’s title. They were held on a four-year cycle, like the Olympic Games, but were offset from the Olympic schedule.

The games are sometimes grouped with midsummer celebrations because ancient Greek festivals followed religious calendars and seasonal cycles, but they were not simply a generic Greek midsummer holiday. The Olympic Games honored Zeus at Olympia, whereas the Pythian Games honored Apollo at Delphi.