In the seventh speech of Isaeus (§27) there occurs one of the few references to the Pythais, the occasional sacred pilgrimage from Athens to Delphi. This allusion used to be misinterpreted as a reference to the Pythian games, and was used to date the speech. Elsewhere (§38) the same speaker alludes to the system of trierarchies organised in symmories, and as this was established by the law of Periander, dateable to 357–6 B.C., it is clear that the speech was delivered after that date. Hence it was taken that the Pythian games intended were those of 354 B.C., as it seemed unlikely that Isaeus' speech was written as late as the subsequent festivals of 350 B.C. or thereafter.

In 1918 Boethius (Die Pythais, pp. 19 ff.) showed conclusively that the reference was not to the celebration of the Pythian games, but to the special form of sacred pilgrimage known as the Pythais. In the fourth century B.C. this procession used to be dispatched from Athens in response to certain omens of lightning-flashes which were looked for on certain days during the three summer months. The priestly observers were stationed at the precinct of Zeus Asteropaeus near the Pythion, and watched in the direction of Harma on Mount Parnes. Only if the proper omen was reported, was the Pythais prepared and sent off. But the irregularity and infrequency of the manifestation had become proverbial.