Durga Puja festival marks the victory of Goddess Durga over the evil buffalo demon Mahishasura. Thus, Durga Puja festival epitomises the victory of Good over Evil.
Durga Puja is widely celebrated in the Indian states of Assam, Bihar, Jharkhand, Manipur, Odisha, Tripura and West Bengal, where it is a five-day annual holiday. In West Bengal and Tripura, which has a majority of Bengali Hindus, it is the biggest festival of the year. Not only is it the biggest Hindu festival celebrated throughout the state, it is also the most significant socio-cultural event in Bengali Hindu society. Apart from eastern India, Durga Puja is also celebrated in Delhi, Uttar Pradesh, Maharashtra, Gujarat, Punjab, Kashmir, Andhra Pradesh, Karnataka and Kerala. Durga Puja is also celebrated as a major festival in Nepal where 91% is Hindu. It is also celebrated in Bangladesh where the 8% population is Hindu. Nowadays, many diaspora Bengali cultural organisations arrange for Durgotsab in countries such as the United States, Canada, United Kingdom, Australia, Germany, France, The Netherlands, Singapore and Kuwait, among others. In 2006, a grand Durga Puja ceremony was held in the Great Court of the British Museum.
The prominence of Durga Puja increased gradually during the British Raj in Bengal. After the Hindu reformists identified Durga with India, she became an icon for the Indian independence movement. In the first quarter of the 20th century, the tradition of Baroyari or Community Puja was popularised due to this. After independence, Durga Puja became one of the largest celebrated festivals in the whole world.
Durga Puja also includes the worship of Shiva, who is Durga's consort (Durga is an aspect of Goddess Parvati), in addition to Lakshmi, Saraswati with Ganesha and Kartikeya, who are considered to be Durga's children. Worship of mother nature is done, through nine types of plant (called "Kala Bou"), including a plantain (banana) tree, which represent nine divine forms of Goddess Durga. Modern traditions have come to include the display of decorated pandals and artistically depicted sculptures (murti) of Durga, exchange of Vijaya greetings and publication of Puja Annuals.
Origin of the autumnal ceremony "Sharadiya"
The actual worship of the Goddess Durga as stipulated by the Hindu scriptures falls in the month of Chaitra, which roughly overlaps with March or April and is called Basanti Durga Puja. This ceremony is not observed by many and is restricted to a handful in the state of West Bengal. The more popular form, which is also known as Sharadiya (Autumnal) Durga Puja, is celebrated later in the year with the dates falling either in September or October. Since the Goddess is invoked at the wrong time, it is called "Akaal Bodhon" in Bengali.
There is also a mythological belief that Lord Ram, who was a Durga worshiper, worshiped Goddess Durga before going for a war with Ravana. Rama had performed "Chandi Homa" and invoked the blessings of Durga, who blessed Rama with secret knowledge of the way to kill Ravana. On the day of Ashvin Shukla Dashami, Rama's party found Sita and defeated Ravana. This day is thus also celebrated as Dussehra.
Durga Puja and Mantras
Mantras, the integral part of Durga puja are accompanied by the rhythmic beatings of the dhak, smell of the incense sticks, "dhoono", and flowers. These create the atmosphere of Durga puja. Chanting of mantras in Sanskrit is essential part of the Durga Puja festival. This is a part of Durga Puja's Pushpanjali Mantra:

“ 	
Om Jayanti, Mangala, Kali,
            Bhadrakali, Kapalini. 
Durga, Shiba, Kshama, Dhatri, 
       Swaha, Swadha Namahstu Te.
Esha Sachandana Gandha Pushpa
         Bilwa Patranjali,
 Om Hrring Durgaoi Namah
                             ”

Durga Slokas (which is also known as Devi Mantra) praises Durga as symbol of all divine forces. According to the sloka, Durga is omnipresent as the embodiment of power, intelligence, peace, wealth, morality etc. A part of Durga Sloka is as follows:

 “ 
Ya Devi sarva bhuteshu 
          Matri rupena samsthita
Ya Devi sarva bhuteshu 
          Shakti rupena samsthita
Ya Devi sarva bhutesu  
          Shanti rupena samsthita
Namestasyai Namestasyai
         Namestasyai Namoh Namah 	

                              “  

Translation:
“ The goddess who is omnipresent as the personification of universal mother
The goddess who is omnipresent as the embodiment of power
The goddess who is omnipresent as the symbol of peace
I bow to her, I bow to her, I bow to her ”