Fasting is one of popular ritual practiced in Hinduism. It is done in order to express your devotion to God. According to Vedic literatures, God resides in every human body. When you are fasting, you are expressing your gratitude towards the God within yourself. But if you starve yourself, the God inside you will not be pleased. Though this practice is purely a personal choice, it is a good practice which makes you feel cleansed of your mind and body.
There is no absolute rule for fasting in modern Hinduism unless you are performing a specific yaga or ritual which in your case is for Lord Shiva on every Monday. In this ritual you may go on fasting for entire day or half day. You can also consume milk or water. This is purely a personal choice. In past, devotees used to fast in order to express their faith in God and as a process to purify their souls, as a good karma. This ritual has been taken to extreme level by Vedic and spiritual saints/gurus/sanyasi who kept fasting for days until their soul is met with the God.
In ancient Vedas, fasting is also used as punishment. The head of the house may keep his family members for fasting if they commit any sins. Some kings used to punish thieves or other citizens of his kingdom through making them starve, in other words keep them on fasting. This indicates that fasting is atonement for the sins people commit as fasting purifies their sins and souls.
In modern era, fasting is considered as considered as a ritual which expels your bodily sins and finds a way to rekindle with spirituality. So if you want to do fasting remember that you are pleasing your inner God and also expiating your wrong deeds. Generally fasting means you should avoid taking any forms of food (solid and liquid), avoid drinking water and sway away from sexual pleasure. You can be selective in this.
According to Hindu literature, a day starts with a particular time called "Brahma Muhurtha" which is early in the morning nearly at 4:30 AM. This time indicates that charioteer of the Sun God, Aruna who rises above the horizon. This can be sighted in sky by seeing the planet Venus as a bright star. This happens just before sunrise. While the day ends after the sunset. Although the time is not specific when the day ends, it is usually considered as the time of darkness where you cannot see any other person, implying the sunset. In ancient literature a day is divided into Praharas and Muhurtas. Praharas is a brief division of day into two parts, four praharas for day and four praharas for night. These praharas overlap with muhurtas which is more precise division of day. Though this needs elaborate explaination about how to calculate the time in day, I will keep it simple. A day starts with 'Brahma Muhurtha' and ends with sunset. The next new day is considered at the time of Brahma Muhurta only.
So you can start eating after sunset after offering the prayer to God.
Dr. V. Shashikanth