Gudi Padwa – the day of creation of universe

When Celebrated –
It is celebrated on the first day of the Chaitra month of Hindu calendar and mark the beginning of the New Year. (March or April)

Why Celebrated - Significance

1. It signifies the arrival of spring and to the reaping of Rabi crops.
2. It commemorates the coronation of Rama in Ayodhya after his victory over Ravana.
3. It is the mythical day on which Hindu god Brahma created time and universe.
4. It also said that Vishnu Bhagwan incarnated as Matsya, the fish on this day. 
5. It is first day of Vasanta Navaratri.

How & Where Celebrated

1. It is main festival in Maharashtra and celebrated in neighbouring states also. It is called Gudi Padwa in Maharashtra, Amvatsar Padvo in Konkan & Goa, Yugadi in Karnataka, Ugadi in Andhra Pradesh & Telangana, Navreh or Navreh amongst Kashmiri Pandits and Cheti Chand among the Sindhi people, Sajibu Nongma Panba Cheiraoba in the North-East state of Manipur, as Thapna (Staphana) by Rajasthani and as starting of New Vikram Samwant Year in Haryana.
2. People decorate their houses with mango leaves and 'rangoli' designs, and pray for a prosperous new year.
3. Traditionally, in Maharashtra, they prepare special dish by mixing bitter neem leaves and sweet jaggery. It reminds that life is mix of bitter and sweet experiences and bitter ones are good. They also prepare dishes like Puran Poli, Shrikhand, Aam Panha (refreshing mango drink), Gulab Jamun and the dough for Puri.
4. Numerous Gudi (or Gudhi) arrangements are hoisted on every household. It is a bright colorful silk scarf-like cloth tied at the top of a long bamboo. On top of it, one or more boughs of neem leaves and mango leaves are attached along with a garland of flowers. This arrangement is capped with a silver, bronze or copper pot (handi or kalash) signifying victory. Gudi means flag of victory marked with Swastika. Padwa means crop in Sanskrit.

 
References:
https://en.wikipedia.org/wiki/Gudi_Padwa