Saptami is the seventh tithi of the bright fortnight, a Bhadra or wholesome tithi in the fivefold grouping. Its presiding deity is Surya, the Sun, and the seventh tithi carries solar observances: Ratha Saptami, in the bright fortnight of Magha, marks the Sun's turn toward the northern hemisphere and is kept as a kind of solar new year and day of purification. By this tithi the waxing Moon stands near its first quarter, half-lit in the evening sky. The panchanga reads Saptami as generally auspicious. These are the conventions of the calendar tradition, and the solar association of the seventh day sits alongside the lunar reckoning that defines the tithi itself, a reminder that the Hindu luni-solar calendar tracks both lights at once.
Ratha Saptami takes its name from the image of Surya's chariot, drawn by seven horses representing the colours of visible light and driven by the charioteer Aruna, and the observance of bathing before dawn on this tithi is explained in some sources as a ritual re-enactment of that chariot's turn toward the northern hemisphere. The pairing of a solar festival with a fundamentally lunar unit of time, the tithi, is a reminder that the Hindu calendar is properly luni-solar rather than purely lunar: months and tithis are reckoned from the Moon, but the year as a whole is kept aligned to the solar seasons through the insertion of an extra intercalary month, Adhika Masa, roughly once every three years, without which the lunar months would drift steadily out of step with the seasons over time.