Why is the ninth month called September?
1660s, from be- + dizen "to dress" (1610s), especially, from late 18c., "to dress finely, adorn," originally "to dress (a distaff) for spinning" (1520s), and evidently the verbal form of the first element in distaff.
It is remarkable that neither the vb., nor the sb. as a separate word, has been found in OE. or ME., and that on the other hand no vb. corresponding to dizen is known in L.G. or Du. [OED]