Explanation:

The full moon day is also known as "Purnima" in Hindu mythology. It is one of the most important and auspicious days in the lunar calendar. Purnima is derived from the Sanskrit word "Purnamasi" which means "full moon."

Significance of Purnima:
Purnima holds great significance in various cultures and religions. It is believed to be a time of spiritual growth, enlightenment, and divine blessings. Many religious rituals and festivals are celebrated on this day.

Importance in Hinduism:
In Hinduism, Purnima is considered sacred and is associated with various deities and celestial beings. It is believed that on this day, the moon is in its full glory and radiates positive energy. Some of the major Hindu festivals celebrated on Purnima are:
- Raksha Bandhan: A festival celebrating the bond between brothers and sisters.
- Guru Purnima: A day to honor and express gratitude towards spiritual and academic teachers.
- Sharad Purnima: The full moon day in the month of Ashwin, celebrated with dance and music.

Other Cultures and Beliefs:
Purnima is not just significant in Hinduism but also in other cultures and beliefs. Some examples include:
- Buddhist Purnima: Celebrated as Vesak or Buddha Purnima, commemorating the birth, enlightenment, and death of Gautam Buddha.
- Jain Purnima: Celebrated as Paryushana, a time of self-reflection and spiritual purification for Jains.
- Chinese Purnima: Celebrated as Mid-Autumn Festival, where families gather to admire the full moon and enjoy mooncakes.

Conclusion:
The full moon day is known as Purnima and holds immense cultural, religious, and spiritual significance. It is a time for celebration, reflection, and seeking blessings. Purnima is a beautiful phenomenon observed by people worldwide, connecting them with nature and the celestial realm.