The new-moon in Feb-March (as Atma currently got aware) is known as "Sivas great night". Atma is also pretty sure, as far as seen, that this day relates also to the union of the Gotra Brahmin (Kambo clan from former north of Magada, Cambo, today Afganistan), Kauṇḍinya, with the Mountain princes and Naga Queen, Soma.
Although still not sure, no clear sign, about the exact year (some points on the year 68), Atma would say that it was 57 (five hundred years after the Buddhas Mahaparinibbantāna)
So 4.3.2057, or 4. March 2600 (after Buddha) would be the 2000 anniversary of Khmer Monarchy.
How ever, this merely worldly anniversary, yet nevertheless with much Dhammic effects, falls on 9. March this year.
Accidentally it's also the National Cultur day this year.