Undecimber

I traveled to the end of time, beyond the 13th moon
Seeking out the largest prime, the year ended too soon
Another month would be sublime, the other side of June
Undecimber, hear my tune

The ancient Roman calendar, Intercalaris Prior
The aging roaming scavenger lets Vulcan share his fire
The vagrant roaring amateur, he preaches ere his choir
Undecimber, now transpire

Undecimber what do you mean?
Literally 11, but to you 13
Undecimber where do you stay?
Between New Year's Eve and New Year's Day
Undecimber what's your Abraxas?
Synodic months and extra taxes
Undecimber when are you seen?
7 years out of every 19

You ventured to the end of days, beyond the 13th sun
Seeking out the largest maze, the extra time was won
Another month would be a phase to cherish what was done
Undecimber, be the one