The seventh day of the week is called Ravivaar. Ravi is another name for the sun. Tradition has long connected this day with Surya Dev, the sun deity.
The sun rising each day is a simple and universal sight. From ancient times, one of the seven days has been dedicated to the sun across many traditions.
The Yuddha Kanda of the Valmiki Ramayana contains the Aditya Hridaya Stotra. In it, the sage Agastya recites a hymn in praise of the sun to Lord Rama. This is considered one of the oldest surviving stotra (hymns) connected with Surya Dev.
The Brahma Purana and other Puranic texts also describe the glory of the sun. According to tradition, many devotees have offered water to the sun on Sundays. The Konark Sun Temple in Odisha and the Modhera Sun Temple in Gujarat are ancient sites connected with this same tradition.