I would consider "augstmond" completely equivalent to "augst".
What we're seeing is an interplay between two systems. On the one hand, you have the Latin month names, which were widely used in written sources. These have the advantage of being standardized, but they also feel like untransparent, foreign words to laypeople. In Germanic dialects, a range of month names was still in use, derived from the earlier lunar calendar.
The sources we are encountering reintroduce some of these dialectical names into writing, but often mix them with Latin names. The ones we have in the Drive doc show a similar pattern:
It's abundantly clear that the VM uses the Latin month names. But "aberil" and "augst" may be subtle indications of familiarity with the dialectical forms.
What we're seeing is an interplay between two systems. On the one hand, you have the Latin month names, which were widely used in written sources. These have the advantage of being standardized, but they also feel like untransparent, foreign words to laypeople. In Germanic dialects, a range of month names was still in use, derived from the earlier lunar calendar.
The sources we are encountering reintroduce some of these dialectical names into writing, but often mix them with Latin names. The ones we have in the Drive doc show a similar pattern:
- March, April, May have a version of the Latin name. March is always "mertz", but April often has "b" or even a vowel insertion.
- June is always a version of "Brachmonat", plow month.
- July is a version of "Hewmonat", haying month.
- August may be "augst". This is a tricky one: it derives from the Latin "Augustus", but derivates like "oogst" in Ducth became synonymous with "harvest". In other words, it's possible that "augst" with omission of the last vowel was seen as one of these agricultural terms, even if it came from Latin.
- September, October, November and December shift around a bit, but they involve the words "Herbst" (autumn or harvest) and Winter. In the "cleanest" form, these months are First Autumn, Second Autumn, First Winter and Second Winter.
It's abundantly clear that the VM uses the Latin month names. But "aberil" and "augst" may be subtle indications of familiarity with the dialectical forms.