Sinhala and Tamil New Year is one of the biggest festivals that is celebrated by Sinhalese Buddhists and Tamil Hindu people of Sri Lanka. It is usually held on 13th and 14th of April and these two days are considered as national holidays. To celebrate this event Astrologers usually come up with set of times to do certain rituals. 

 Before the New Year arises the Sinhalese Buddhist goes to the temple and this specific time is called the ‘Nonagathaya’. At this time they do not do any other special deeds other than attending religious activities at the temple or at home.
A new year is celebrated when the Sun moves from Meena Rashiya (Pisces) to Mesha Rashiya (Aries) and completes a 12 month period as a whole. Sinhalese farmers and their families give thanks for receiving the harvest from their paddy fields on this occasion and in villages it is done with many rituals and in a very methodical way that can be admired. If you are a foreigner it will be a great experience for you.  
At the dawn of the New Year, Sinhalese families wear new clothes and get together to have milk rice and traditional sweets specially made for the occasion.
Tamils also celebrate this auspicious occasion in a grand way. They attend the Pooja’s in Kovil and dress in colourful clothing to welcome the new year.
This festival is filled with colour and a lot of fun. The youth in villages play many traditional games by getting together at a known place in the village. They will try to climb a poll that is covered with grease, they will eat buns as soon as possible or pillow fight on a poll that is put across in a high place etc. You will be amused by these traditional games and if you love trying out new things you will be tempted to join in as well.
Aluth Avuruddha (The New Year) is a time they make new friendships and spend some quality time with their own families. Women in villages get together and play a big round drum called Rabhana and sing folk songs to entertain the crowdswearing the traditional Sinhalese reddha and hattaya (Blouse and a long cloth wrapped around like a skirt).
If you want to see an interesting Sinhalese New Year, you should attend a New Year Festival bit far from Colombo that follows real traditions.  Tamil New Year is celebrated in the northern part of the country.To get a real feel of how they celebrate their New Year you can visit to north as well.