Sunday is dedicated to Lord Surya

Sunday is dedicated to Lord Surya or Suryanarayana - the Sun God.
Ritual:

After a ritual bath and cleansing of their body and their house and surrounding, devotees offer red color flowers to Surya and apply red sandalwood paste as 'tilak' on their forehead. They keep a fast throughout the day and eat food only once before sunset and avoid salt, oil and fried food items. Alms are also given on that day. Ravivar Vrat or Sunday fasts help in fulfilling ones wishes and it is believed that all kinds of skin diseases get cured.
Color & Gem:

Red and pink are the color of the day and ruby the preferred gem.
Celestial Body:

Sun or Surya rules Sunday.