Thursday is dedicated to Lord Vishnu and Lord Brihaspati, the guru of gods. Hence, this day is called Vrihaspativar, after the apportioned god. Thursday is also popularly known as Guruvar. Devotees listen to devotional songs, such as "Om Jai Jagadish Hare" dedicated to Lord Vishnu on this day.
Devotees adorn yellow colored clothes and offer yellow flowers to propitiate god Vishnu and Vrihaspati. They fast throughout the day and consume food only once which consists of 'chana daal' (Bengal Gram) and 'ghee' (clarified butter). The food color used is yellow. Many offer banana or plantain to god. In Bengal, they worship goddess Lakshmi and elsewhere, people also visit Hanuman temples on Thursdays. Through 'pujas' (worship) and 'vrats' (fast) on Thursdays, it is believed, devotees are blessed with wealth, success, fame, and happiness.
The popular myth is that Lord Vishnu once appears before a devout Hindu disguised as a mendicant to test the devotee on a Thursday. Some believe it was Lord Vrihaspati. At first the devotee neglects his duties towards the holy man and denies him alms and is lethargic but later when he realizes his mistake, he observes a ritual fast on Thursdays and appeases the gods.
Color & Gem:
Yellow is the color of the day and the preferred gems are Yellow Sapphire or Yellow Diamond.