Dog days sometimes used for the lethargic summer markets. Various computations of the dog days have placed their start anywhere from 3 July to 15 August and lasting for anywhere from 30 to 61 days. They may begin or end with the cosmical rising or heliacal rising of either Sirius in Canis Major or Procyon (the "Little Dog Star") in Canis Minor and vary by latitude, not even being visible throughout much of the Southern Hemisphere. Sirius observes a period of almost exactly 365¼ days between risings, keeping it largely consistent with the Julian but not the Gregorian calendar; nonetheless, its dates
Dog days even recorded the belief that the return of the bright star was responsible for bringing heat and fever with it; it was also associated with sudden thunderstorms. In Homer's "Iliad", probably composed in the but representing an earlier tradition, Achilles' approach towards Troy, where he will slay Hector, is illustrated through an extended metaphor about the baleful effects attending the return of Sirius: The rising of Sirius during this period has been calculated as 19 July (Julian). Writing about the same time, Hesiod, however, considered the worst and hottest part of the summer to be the days "before" Sirius returned