Salve
Well, no I do not find it arbitrary at all. The old calendar followed the seasons as signalled by the stars, which was needed back then when people still worked with Nature. We are a more urban society today, one that has disconnected people from Nature, and so I find that making an effort to place our religious celebrations back in sync with Nature to be a valuable exercise.
Yes, the stars are not now where they were two thousand years ago. It is not simply a matter of the colar points having shifted back along the zodiac. That effect results from a wobble in the earth's rotation, at the same time that the tilt of the earth is less than it was before. Something like Alnitah, in Orion's Belt, today culminates at 58.0 degrees, where it was at 44.9 degrees some 4500 years ago. Appearing higher in the sky today, it is going to be observed rising earlier in the relative year. And the relative positions between the stars are now different too. Sirius IIRC rises lower and is more elongated from its earlier position, so instead of four days difference as with the equinoxes it works out around 10 days? difference? So you do have to obseve when the appropriate star rising occurs - it is not a simple adjustment of just shifting one calendar against the other.
The other thing, of course, is that the pontifical calendar was once based on the lunar cycle, which differed from the civil or rustic calendars. It is a little easier to work out the adjustments, but you have to do it every year. this year, it just happens, the lunar cycle is closer to the Gregorian than with the Julian calendar, but the lunar cycle increasingly slips as we progress through the year. In January 2006 the Full Moon fell on the 14th, so only about a half of day off from the Ides of January. Here we are at the first of May (Gregorian), four days after the New Moon, with the Full Moon of May two days off from the Ides. And by December, the Full Moon arriving on the 4th will be nine days off from the Ides.
But then why do we have a religious calendar? Why do we celebrate certain days as festivals? You can make it arbitrary I suppose, but that was not the original intention, and I think you lose some of the meaning of celebrating the Ides by doing it on an arbitrary date rather than by coinciding your celebration with the celestial event of a Full Moon.
A BIG BTW - 1 May marks the anniversary of the Founding of our Societas
FELICES NATALIS SVR
Vale optime et vade in pace Deorum