Einhard. The Life of the Emperor Charles. Chapter 29.
It was after he had received the imperial name that, finding the laws of his people very defective (the Franks have two sets of laws, very different in many particulars), he determined to add what was wanting, to reconcile the discrepancies, and to correct what was vicious and wrongly cited in them. However, he went no further in this matter than to supplement the laws by a few capitularies, and those imperfect ones; but he caused the unwritten laws of all the tribes that came under his rule to be compiled and reduced to writing. He also had the old rude songs that celebrate the deeds and wars of the ancient kings written out for transmission to posterity. He began a grammer of his native language. He gave the months names in his own tongue, in place of the Latin and barbarous names by which they were formerly known among the Franks. He likewise designated the winds by twelve appropriate names; there were hardly more than four distinctive ones in use before. He called January, Wintarmanoth; February, Hornung; March, Lentzinmanoth; April, Ostarmanoth; May, Winnemanoth; August, Aranmanoth; September, Witumanoth; October, Windumemanoth; November, Herbistmanoth; December, Heilagmanoth. He styled the winds as follows; Subsolanus, Ostroniwint; Eurus, Ostsundroni; Eurauster, Sundostroni; Auster, Sundroni; Austro-Africus, Sundwestroni; Africus, Westsundroni; Zephryus, Westroni; Caurus, Westnordroni; Circius, Nordwestroni; Septentrio, Nordroni; Aquilo, Nordostroni; Vulturnus, Ostnordroni.