Thursday is dedicated to Hindu God Vishnu, Brihaspati
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.
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 pooja 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.