Janmashtami is a Hindu festival that celebrates the birth of Lord Krishna, an incarnation of Lord Vishnu. Fasting on Janmashtami is a common practice among devotees, and it holds significance for several reasons:

1. Spiritual Cleansing: Fasting is seen as a way to purify the mind and body. By abstaining from food and water, devotees believe they can cleanse themselves of impurities and focus their thoughts on spiritual matters. It is a form of self-discipline and self-control.

2. Devotion and Surrender: Fasting on Janmashtami is an expression of devotion and surrender to Lord Krishna. It is a way for devotees to show their dedication and commitment to the divine. By willingly giving up physical nourishment for a day, devotees demonstrate their love and reverence for Lord Krishna.

3. Commemorating Lord Krishna's Leelas: Janmashtami celebrates the birth of Lord Krishna, who is believed to have performed various divine and playful acts (leelas) during his time on Earth. Fasting is a way for devotees to remember and connect with the significance of Lord Krishna's birth and his divine activities.

4. Observance of Austerity: Fasting is considered an act of austerity in many religious traditions. It is believed to bring about spiritual growth, detachment from material desires, and increased focus on spiritual pursuits. By observing a fast on Janmashtami, devotees aim to cultivate qualities of simplicity, humility, and detachment.

5. Symbolism of Krishna's Childhood: Lord Krishna's childhood is characterized by his love for butter and dairy products. Fasting until midnight on Janmashtami symbolizes the wait for Krishna to be born and the anticipation of the midnight hour when he made his divine appearance.

It's important to note that fasting practices may vary among individuals and different regions. Some people may observe a complete fast without consuming any food or water, while others may follow a partial fast by consuming only fruits, milk, or specific vegetarian dishes. The specific fasting guidelines may be influenced by cultural traditions and personal preferences.

Krishna Janmashtami is the celebration of the birth of Shri Krishna Ji, the eighth Avatar of Shri Vishnu Ji. The common observances of the festival include praying, fasting, and the recitation of the Holy Books. The importance of Shri Krishna Ji is considered to be the miracles he performed throughout his life and the knowledge given in the Holy Geeta Ji. But, does this Krishna Janmashtami celebration and fasting really comply with Holy Geeta Ji?

Answer is No because on this event, People keep fast throughout the day and break it after midnight, since it is believed that Shri Krishna Ji was born in the midnight hour. But, the irony is that fasting is forbidden in our Holy Geeta Ji.

In our Holy Geeta Ji Adhyay 6 Shloka 16 :

Na, ati, ashnatH, tu, yogH, asti, na, ch, ekaantam, anashnatH,
Na, ch, ati, swapnsheelasya, jaagrtH, na, ev, ch, Arjun ||16||

Translation: Oh Arjun, the (yogH) bhakti for attaining that Purna Parmatma is (na ekaantam) neither successful by sitting in a lonely place on a special seat or position, nor of a person who eats too much, and nor of a person who does not eat at all, i.e., keep fasts, nor of a person who sleeps too much, and nor of a person who forcefully keeps awake.

Kabir, guru bin kahu na paya gyana, jyon thotha bhus chhade mudh kisana।
Guru bin bed padhe jo prani, samjhe na saar rahe agyani।।

So, Firstelly we have to understand what all our holybook is saying to us to do devotion, then by implementing them, we should have our precious human birth in devotion and to attain salvation.