Oh, this is interesting. Why April 14th? Because it's a sidereal calendar!
~1900 years ago the New Year was on the equinox, but the equinox has since moved three and a half weeks.
The Gregorian cale...
There's a problem with this explanation. The Kollam Era, which is the era of the calendar, starts in the year 825 CE, which is much more recent than 1900 years. Was there an existing older equinox celebration that it was attached to, or is the explanation bogus and the almost-equinox date a coincidence?