month names

[nablator]

Unfortunately, I don't understand it's meaning.

The four circles are quite readable, the one in the middle is more difficult.
ista figura deseruit anno bisextilem - this figure is used for a leap year
ista figura deseruit primo anno post bisextilem - this figure is used for the year after a leap year
ista figura deseruit secundo anno post bisextilem - this figure is used for the second year after a leap year
ista figura deseruit tertio anno post bisextilem - this figure is used for the third year after a leap year

I also don't understand the 13th month (last in the row).

Embolis(m)us: (Medieval uncommon) insertion; (in literary work)