The story
The Sun's chariot has one wheel, seven horses and a driver with no legs, and today it turns toward the north. The seven horses are read as the seven colours, or the seven days of the week, or the seven metres of Vedic verse.
The day is kept as the Sun's own birthday and as the true beginning of the warm half of the year — Makara Saṅkrānti is the astronomical turn, Ratha Saptamī the ritual one.
It is also called Acalā Saptamī, the unmoving seventh, and a bath taken at sunrise today is held to count for many.