"Februarius" was the only month in the pre-Julian calendar to have an even number of days, numbering 28. This was mathematically necessary to permit the year itself to have an odd number of days. Ancient sources derived "Februarius" from "februum", a thing used for ritual purification. Most of the observances in this month concerned the dead or closure, reflecting the month's original position at the end of the year. The Parentalia was a nine-day festival honoring the ancestors and propitiating the dead, while the Terminalia was a set of rituals pertaining to boundary stones that was probably also felt to reinforce the boundary of the year.