Months names are all one off?

[ReneZ]

Currently each sign of the Zodiac starts around the 20th of one month and ends around the 20th of the next month.  I suppose that these dates were chosen so that the solstices and equinoxes fall on sign boundaries. Is that correct?

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Anyway, it seems that the VMS month names were written after the Zodiac sync.

Welcome Jorge!

I cannot explain this offset, but one thing is certain: the spring (vernal) equinox was always supposed to be the point where the sun enters the sign of Aries. Precession only affects the relationship between the sign and the constellation after which it was originally named, but this plays no role here.

The date when this happens had moved back about 10 days (to around 12 March -ish) by the 15th centuty, until this was fixed by the Gregorian reform. But also that does not explain the offset. It even makes things worse: only 1 third of the month written in the MS coincides with the sign where it is written.

I may misunderstand what you mean with "zodiac sync" but there was no such sync.

Not long ago, there was a post by @nablator about the very different French calendars in the past (IIRC), which may explain, but still, the main books of hours, expecially also from France, had (e.g.) July correspond with Leo, not with Cancer.

One correspondence that works is that the first day of each month (as written in the MS) is during the sign where it is written.

And less seriously, but still true: the month names here in Thailand are named after zodiac signs, and they follow the correspondence in the Voynich MS...