In Sanatan Dharma every day is dedicated to some deity. According to Hindu mythology, each day is dedicated to some god. So, if you especially want to worship all the incarnations and forms of the Lord, you get a full day to worship the Lord. Each day of the week is dedicated to a particular God and a particular God is worshipped. For example Monday is dedicated to Lord Shiva and Tuesday is dedicated to Lord Hanuman.
If there is any fast and festival on any day which depends on the constellation. So that particular deity is also worshipped. For example, Goddess Parvati is worshiped on every Tuesday in the month of Sawan. Lord Vishnu is worshiped on the day of Ekadashi. Lord Shiva is worshiped on the amavasya day. There is a law to worship Lord Ganesha on the day of Chaturdarshi.
Lord Shiva is worshiped on Monday. On Monday, devotees fast for the whole day and can have one meal at a time.
Lord Hanuman is worshiped on Tuesdays. Hanuman ji is said to be the lord of power and Lord Rama is a devotee. Hanuman ji is said to be an incarnation of Lord Shiva.
There is a law to worship Lord Ganesha on Wednesday. The best day to worship Lord Ganesha is the remover of all obstacles.
Lord Vishnu is worshiped on Thursday. It is also a day to worship Goddess Lakshmi, the consort of Vishnu. It is said that Thursday is an auspicious day to worship the god of luck and wealth.
On Friday, Jagat Janni Mata Parvati is worshipped and Mata Durga, Santoshi Ma and all her incarnations are worshipped. There is also a law of worship and fasting of Mahalakshmi on this day. Fasting for the whole day and can take one meal at a time.
There is a law to worship Lord Shani Dev on Saturday. Navagrahas, Hanuman and Goddess Kali are also worshiped on this day. Shani Dev removes bad effects. Fasting for the whole day and can take one meal at a time.
Sunday is the day of Lord Surya or Sun God, also known as Lord Suryanarayan. Those observing the fast are allowed to have one meal on this day and that too before sunset.