What percentage of the world's population celebrates New Year's Day on January 1?

This is basically a question of what percentage of the population resides in countries using (our) Gregorian calendar, which is "a vast majority": "Today, a vast majority of countries use the Gregorian calendar as their sole civil calendar. Countries which haven't adopted the Gregorian calendar are Saudi Arabia, Ethiopia, Nepal, Iran and Afghanistan. Some countries use other calendars alongside the Gregorian calendar: India, Bangladesh, Israel and Myanmar; other countries use a modified version of the Gregorian calendar: Sri Lanka, Cambodia, Thailand, Japan, North Korea and Taiwan.

The nearly universal civil adoption of the Gregorian calendar hasn't been followed by most religious organizations, which have often retained their own calendars. These alternate calendars are used in many regions of the world today to mark cycles of religious and astrological events." -from https://en.wikipedia.org/wiki/Gregorian_calendar

Based on this, and on the figures from http://simple.wikipedia.org/wiki/List_of_countries_by_population, the countries not celebrating the Gregorian New Year contain 82,101,998 (Ethiopia) + 76,991,000 (Iran) + 32,358,000 (Afghanistan) + 27,136,977 (Saudi Arabia) + 26,620,809 (Nepal) = 245,208,784 people out of a world population estimated at the same site to be 7,059,900,000: I calculate this to be 96.5267% of the world's current population living in countries that recognise tomorrow (FT time) as 1 January 2013, New Year's Day.

This cannot account for each individual's choices to celebrate it, or not, for religious or other reasons. Further, some of these individuals may also (or instead) choose to celebrate the Julian calendar's 1 January, 13 days later, regardless of its "official" national status.

The above answer makes no mention of China. Although China uses the Gregorian calendar, Chinese New Year is celebrated, not on January the first, but on a non-fixed date between late January and late February.

https://en.wikipedia.org/wiki/Chinese_New_Year

Since China itself has about 19% of the world's population, and Chinese people in many parts of the world observe Chinese New Year, the percentage of the population celebrating New Year's Day on January the first is more like 75% (after allowing for sportsherald's 3.5% also).

*supplementary note* I did not mention China, because it is part of the majority, not an exception. China officially recognises/uses the Gregorian calendar; "Upon its foundation in 1949, the People's Republic of China continued to use the Gregorian calendar with numbered months, but abolished the ROC Era System and adopted Western numbered years." -again from https://en.wikipedia.org/wiki/Gregorian_calendar

As I said earlier, "This cannot account for each individual's choices to celebrate it, or not, for religious or other reasons."