Technically speaking it really should be called Buddhist New Year. Why? Because for Theravada Buddhism, it is widely accepted that Buddha attained Enlightenment on the year 543 BCE. It is from that date the Buddhist year begins. Hence the current Buddhist year 2556 which is equivalent to the year 2013 (that is, 2013+543=2556). The Buddhist Year is often used in tandem with the common international year on official government documents in many Theravadan countries. Admittedly, the year of Enlightenment is debateable and somewhat controversial as others believe it to be a different year.
Theravadin Buddhist countries, as well as pocket communities of Theravadin Buddhists across the world, will celebrate the new year on the 6th day of the waxing moon of the 5th month of the lunar calendar, which according to the common era and calendar falls generally in the middle of April.
As it so happens, in each country that happens to celebrate this they tend to appropriate the celebration and call it their own. Hence you hear it often being called Lao New Year, Thai New Year, Khmer New Year, etc. Truth is, all the major Therevada Buddhist countries celebrate it - they include Laos, Thailand, Myannmar, Cambodia, Nepal, Sri Lanka, ethnic communities in Canada, France, America, Australia, and many more. Therefore, ideally it should be called Buddhist New Year. Though again some may argue not all Buddhist celebrate it at this particular time.