Menarche
"Mensis" is the Latin term for "month." It is commonly used in the medical field to refer to a woman's monthly menstrual cycle.
the answer is 'onset'
men is the prefix of mensis, if you ever have trouble with prefixes, most prefixes are the very first syllable of a word
Menarche
Onset
The term menarche describes onset of menstruation. This is usually charachterisic of age 12 or 13.
Mensis October.
Month.
for mensis
One example is that late onset of pubity in someone who is slow in developing from a child into an adult.
(Mensis) Februarius.
mensis horribilis