The Vulcanalia (which is essentially a term relating to a fire celebration taking place in late August; it is unlikely that the classical Vulcanalia was anything more than a vague date to Martin, since the actual Roman celebration had long been done away with) will be dealt with in another post. But it is the notion that the Heathens would practice some observance of the kalends that is of interest. Much confusion lies in the fact that the writer is composing in Latin, and as such is also using Latin conventions for such things as deity names and calenderical references. It must always be asked, when he speaks of Mercury (for instance) whether he is speaking of the Roman Mercurius or the Germanic Odin, who was associated with the Roman God.
Historically, the Kalends was the first day of the month. What brings in a measure of confusion is the fact that the definition of when a month began had changed from the begining of Rome to the 6th century. At Rome's foundation, the calendar was a lunar one, and the kalends marked the New Moon. By the time of the Imperial period, the calendar we know had been mostly introduced, and the month-names with which we are familiar had been well established. (July and August, for example, were named after Julius Caesar and Caesar Augustus, respectively.)
That begs the question, though; when Martin spoke of the Heathens observing the kalends, did he mean the first day of the calendar month, or did he mean the new moon? I think the answer (such as we have) lies in the politics of Gaul in the late 6th century, when Martin was writing.
Clovis I had only been baptized 80 years before, and although most of the Franklish aristocracy had converted with him, such things were notoriously slow to make their way into the beliefs and practices of the common folk. Martin, writing in Latin, would have used the term kalends to refer to the beginning of the month, no matter how the beginning of the month was actually reckoned. The question becomes, how did the common folk of the Germanic peoples actually figure out the beginning of their months?
Look to Alvissmal:
Mani heitir medh monnum,
en mylinn medh godhum,
kalla hverfanda hvel helju i,
en skin dvergar,
kalla alfar artala.
Tis hight Moon among men,
and Mill among gods
called Rolling-Wheel in Hel.
Hasty by giants,
Shining One by dwarves,
Called by elves, Year-Teller.
And here we have our answer, and it all falls into place.
Martin was writing for a popular audience. His letter was clearly intended to be read from the pulpit to the fallen masses. He would have used the term kalends in a way that was significant to them; not using the Roman calendar (which was solar in nature), but the Germanic lunar calendar of the Frankish peasants to whom he was speaking. The kalends was the new moon.
The Frankish Heathens in Gaul in the late 6th century were observing the new moon. Let us do no less in our reconstruction of their faith in the Gods. Up soon... how?