Wikipedia says of Songkran, the Thai New Year (Moha Songkran is the Cambodian name of the first day of the new year celebration) : "The date of the festival was originally set by astrological calculation, but it is now fixed."
Contrast this with Chinese New Year and Vietnamese Tet, which also follows the lunar calendar. Why does Chinese New Year and Tet fall in January or February and not April? According to wikipedia: "The festival traditionally begins on the first day of the first month (Chinese: 正月; pinyin: zhēng yuè) in the Chinese calendar and ends on the 15th; this day is called Lantern Festival."
If you understood that, great. I don't--why is the first day of the first month in Jan/Feb and not any other month? I am sure there is some logic to it but it is too complicated for me to understand. You can have a go here:
"The zodiac sign which the sun enters during the month and the ecliptic longitude of that entry point usually determine the number of a regular month. Month 1 (正月 zhēngyuè), literally means principal month. All other months are literally numbered, second month, third month, etc."Wikipedia also explains Chinese New Year should always be on the second new moon after the winter solstice:
"An exception will occur in 2033-2034, when the winter solstice is the second solar term in the eleventh month. The next month is a no-entry month and so is intercalary, and a twelfth month follows which contains both the Aquarius and Pisces solar terms (deep cold and rain water). The Year of the Tiger thus begins on the third new moon following the Winter Solstice, and also occurs after the Pisces (rain water) jieqi, on February 19.Okaaaay. After a while, like most traditions and customs, people forget or fail to understand how they came to be, and just accept them, whether sensible or not, simply because things were done that way for as long as people can remember.
"Another occurrence was in 1984-85, after the sun had entered both Capricorn at 270° and Aquarius at 300° in month 11, and then entered Pisces at 330° during the next month, which should have caused it to be month 1. The sun did not enter any sign during the next month. In order to keep the winter solstice in month 11, the month which should have been month 1 became month 12, and the month thereafter became month 1, causing Chinese New Year to occur on 20 February 1985 after the sun had already passed into Pisces at 330° during the previous month, rather than during the month beginning on that day."