Month names collection / metastudy

[Koen G]

For the genitive, I think one would expect KL Marcii, as the occurrence at the bottom, not Marcius.
But I agree that Septembris makes sense as a genitive here

I was looking at the other forms in the MS, most of which follow the "KL genitive" formula: julii, junii, februarii and all the -bris months. The genitive of Aprilis is identical to the nominative.

In practice, Latin month names appear in the genitive very often: the month of..., the 6th day of... So I understand why this can be confusing, as it must have been to some medieval scribes.