A later Council decreed that the date of the Vernal Equinox, which varies a day or so over time, would be standardized on March 21st--an extremely close if not exact approximation of the actual Vernal Equinox--on the then current secular calendar that was a revision of an older one affected by Julius Caesar.
I am not aware of any such "later council." The date of March 21st was never "decreed" by an ecumenical council. It was eventually agreed on everywhere because the Alexandrine computus, in its final form, was agreed on everywhere. And this happened because the Egyptians had a reputation for being the best astronomers, and because the March 21 date, of the available alternatives, agreed best with observations. We find both grounds in Ceolfrid's letter to the Pictish king Nechtan (around A.D. 700):
Aequinoctium autem, iuxta sententiam omnium orientalium, et maxime Aegyptiorum, qui prae ceteris doctoribus calculandi palmam tenent, duodecimo kalendarum Apriliumdie provenire consuevit, ut etiam ipsi horologica inspectione probamus.
Which being interpreted means
The equinox, according to the teaching of all the Easterners, and above all the Egyptians who hold the palm as the foremost teachers of computation, tends to fall on XII Kal. April. (March 21st); as also we demonstrate by means of instrumental measurements.
And finally, the Alexandrine computus won the day because, of the alternatives available to Christians in the period 300-800, it was the most accurate. (The more-accurate Rabbinic calendar stabilized near the end of this period but does not seem to have been well-known among Christians.)
Of course, these factors no longer apply. The equinox no longer tends to fall on March 21 Julian; instrumental measurements support using a more accurate approximation of March 21st Gregorian; and the science of mathematical astronomy is well-known around the world; Egypt no longer "holds the palm" in this field.
And this only refers to the solar discrepancy. I haven't even gotten to the egregious lunar discrepancy in the Julian paschalion. But anyone who has clear skies over the next two weeks will be able, in Roger Bacon's words, to "see the error in the sky" by comparing the moon's appearance on 21-22 August, 2013 (the Gregorian full moon) to her appearance on 25-26 August, 2013 (the Julian full moon).
Your folk should follow the lead of Finland and go on the Gregorian calendar; or, in the alternative, (as others have already stated) the Milankovitch calendar.