Why Sunday specifically
Sunday (Ravivar) is named directly for Surya, also called Ravi, the sun god — making it the one day of the week where the deity and the day share the same name across most Indian languages. As the source of light, warmth and the basic rhythm that structures every other day, Surya's association with the week's first day fits naturally.
How the day is typically observed
Devotees rise before sunrise, offer water (Arghya) to the rising sun while facing east, and recite the Surya Gayatri Mantra or sing Om Jai Surya Bhagwan. Some observe a Sunday fast, eating only once and avoiding salt, particularly those seeking better health or vitality, which Surya is associated with.
Surya Namaskar
Beyond formal worship, Surya Namaskar (sun salutation) — a sequence of yoga postures performed facing the sun, ideally at sunrise — is one of the most widespread daily practices linked to Surya, practiced as much for physical health as for devotion.
A day to reset the week
As the day that opens the week in most Indian calendars, Sunday's dedication to the sun carries a practical symbolism: just as Surya's rising begins each day, devotion to him on this day is treated as a way of beginning the week itself with clarity and renewed energy.