Ram Navami – the day of birth of Purushottam

When Celebrated
Ram Navami is celebrated on the ninth day of the Chaitra (March–April) of Hindu month.  This coincides with Vasantha Navratri.

Why Celebrated - Significance

1. It is birthday of Shree Rama, incarnation of Bhagwan Vishnu who was born to Kaushalya and Dasharatha in Ayodhya.
2. In South India, since the auspicious day is also considered as the marriage day of Rama and Sita, temple priests perform a wedding ceremony called "Kalyanotsavam".
3. Shree Rama is especially important to Vaishnavism. He is the central figure of the ancient Hindu epic Ramayana.
4. This festival celebration indicates the victory of goodness and defeat of evil power and establishment of the Dharma after removal of Adharma.
5. Shree Rama is called Purushottam as he idealize as son, step-son, brother, husband and king. He teaches loyalty, sacrifice of his comforts for father’s dignity & Kingdom, one wife principle, protect your wife, care for brothers, humility etc.   

 
How & Where Celebrated

1. It is celebrated by reading and listening Shree Ramcharitra Manas, the Hindu epic, written by Sri Tulsidas.
2. Akhand Paath of the Ramacharitamanas for nine days is organised alongwith religious bhajan, havan, ritual kirtan and distribution of prasad after the puja and Aarti on 9th day.
3. Temples are decorated, traditional prayers are organised and fruits and sweets are distributed in order to get their problems removed and for lot of prosperity and success.
4. It is considered auspicious to undertake a fast on this day.
5. A grand celebration of the Ram Navami takes place by the thousands of devotees in the Ayodhya, Uttar Pradesh; Sitamarhi, Bihar; Rameswaram, Tamil Nadu, Bhadrachalam, Andhra Pradesh etc.
6. At some places (like Ayodhya, Varanasi and etc), the Rathayatra means the chariot procession (Shobha yatra) of the Lord Rama, Mata Sita, Lakshman and God Hanuman is carried out by the thousands of people after taking a holy dip in the sacred river, Sarayu or Ganga.

​Celebrated since when
Shree Rama is stated to have lived in the Treta yuga that their authors estimate existed before about 5,000 BCE.
 
References:

1. https://en.wikipedia.org/wiki/Rama
2. http://www.indiacelebrating.com/festivals/ram-navami/