In Hinduism, each day in a week is dedicated to a particular god in the Hindu pantheon. Thursday, which is Brihaspatiwar or Vrihaspativar, is dedicated to Vishnu and Brihaspati, the Guru of Devas. Thurdsay is also popularly known as Gurubar or Guruvar. Yellow is the color of the day. A fast is observed on the day and food is consumed only once. In some areas, people make it a point to visit Hanuman Temple on Thursdays.
There are numerous stories associated with the reason for observing Upvaas, or fast, on Brihaspativar. Many of the stories are part of local folklore and stories vary from region to region.
Almost all stories suggest that those who perform pujas and Vrats on Thursdays will be blessed with wealth and happy life. In some stories, Lord Vishnu appears in the guise of a Sadhu to test the devotees on Thursdays.
In other stories, it is Brihaspati. First the person in the story denies alms and is lethargic but later realizes his/her mistake and observes Thursday Vrat and pleases the Gods.
Those devotees fasting on the day wears yellow colored dress and offers yellow colored fruits and flowers to Vishnu and Brihaspati. Food is only consumed once and includes Chana Dal (Bengal Gram) and ghee. Basically, yellow colored food is eaten on the day.
In some regions, banana or plantain is worshipped and watered.