The dog days or dog days of summer are the hot, sultry days of summer. They were historically the period following the heliacal rising of the star system Sirius (known colloquially as the "Dog Star"), which Hellenistic astrology connected with heat, drought, sudden thunderstorms, lethargy, fever, mad dogs, and bad luck. They are now taken to be the hottest, most uncomfortable part of summer in the Northern Hemisphere.

The English name is a calque of the Latin dies caniculares (lit. 'the puppy days'), itself a calque of the ancient Greek κυνάδες ἡμέραι (kynádes hēmérai). The Greeks knew the star α Canis Majoris by several names, including Sirius "Scorcher" (Σείριος, Seírios), Sothis (Σῶθις, Sôthis, a transcription of Egyptian Spdt), and the Dog Star (Κῠ́ων, Kúōn). The last name reflects the way Sirius follows the constellation Orion into the night sky.

In Swedish, the dog days are called rötmånaden and mätäkuu in Finnish. Both names mean "rot month". The names stem from the fact that food spoils easily when left out in the heat, and wounds are more likely to get infected and secrete pus during the warm summer months.

Sirius is by far the brightest proper star in the night sky, which caused ancient astronomers to take note of it around the world. In Egypt, its return to the night sky became known as a precursor to the annual flooding of the Nile and was worshipped as the goddess Sopdet. In Greece, it became known as the precursor of the unpleasantly hot phase of the summer. Greek poets 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 8th century BC but representing an earlier tradition, Achilles's approach toward Troy, where he would slay Hector, is illustrated through an extended metaphor about the baleful effects attending the return of Sirius:

τὸν δ᾽ ὃ γέρων Πρίαμος πρῶτος ἴδεν ὀφθαλμοῖσι
παμφαίνονθ᾽ ὥς τ᾽ ἀστέρ᾽ ἐπεσσύμενον πεδίοιο,
ὅς ῥά τ᾽ ὀπώρης εἶσιν, ἀρίζηλοι δέ οἱ αὐγαὶ
φαίνονται πολλοῖσι μετ᾽ ἀστράσι νυκτὸς ἀμολγῷ,
ὅν τε κύν᾽ Ὠρίωνος ἐπίκλησιν καλέουσι.
λαμπρότατος μὲν ὅ γ᾽ ἐστί, κακὸν δέ τε σῆμα τέτυκται,
καί τε φέρει πολλὸν πυρετὸν δειλοῖσι βροτοῖσιν:
ὣς τοῦ χαλκὸς ἔλαμπε περὶ στήθεσσι θέοντος.

Priam saw him first, with his old man's eyes,
A single point of light on Troy's dusty plain.
Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion's Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.
Achilles' bronze gleamed like this as he ran.

The rising of Sirius during this period has been calculated as 19 July (Julian). Writing about the same time, Hesiod considered the worst and hottest part of the summer to be the days before Sirius returned to the night sky. During this period, Sirius was invisible from the earth but it was apparently understood to still be in the sky, augmenting the power of the sun:

ἦμος δὴ λήγει μένος ὀξέος ἠελίοιο
καύματος ἰδαλίμου, μετοπωρινὸν ὀμβρήσαντος
Ζηνὸς ἐρισθενέος, μετὰ δὲ τρέπεται βρότεος χρὼς
πολλὸν ἐλαφρότερος: δὴ γὰρ τότε Σείριος ἀστὴρ
βαιὸν ὑπὲρ κεφαλῆς κηριτρεφέων ἀνθρώπων
ἔρχεται ἠμάτιος, πλεῖον δέ τε νυκτὸς ἐπαυρεῖ:
τῆμος ἀδηκτοτάτη πέλεται τμηθεῖσα σιδήρῳ
ὕλη, φύλλα δ᾽ ἔραζε χέει, πτόρθοιό τε λήγει:
τῆμος ἄρ᾽ ὑλοτομεῖν μεμνημένος ὥρια ἔργα.