In the real old days Samhian was a "moveable" date (as was all Pagan celebrations) Samhain traditionally falls on the day of the midpoint between the Autumnal Equinox and Winter Solstice. Typically that date falls a couple days into November on our modern calendars. It can also be connected with the date that the Zodiac sign Scorpio passes it's mid point in it's transit.
Also remember that when talking about calendars concerning Pagan holidays, the lunar calendar was the typical day - to - day calendar for most people in ancient times just like it is for the religions of Judiasm, Islam, Hinduism, Buddhism, etc.