The Gaulish calendar appears to have divided the year into two halves, the 'dark' half, beginning with the month Samonios (the October/November lunation), and the 'light half', beginning with the month Giamonios (the April/May lunation). The entire year may have been considered as beginning with the 'dark' half, so that the beginning of Samonios may be considered the Celtic New Year's day. The celebration of New Year itself may have taken place during the 'three nights of Samonios' (Gaulish trinux[tion] samo[nii]), the beginning of the lunar cycle which fell nearest to the midpoint between the autumnal equinox and the winter solstice. The lunations marking the middle of each half-year may also have been marked by specific festivals. The Coligny calendar marks the mid-summer moon (see Lughnasadh), but omits the mid-winter one (see Imbolc). The seasons are not oriented at the solar year, viz. solstice and equinox, so the mid-summer festival would fall considerably later than summer solstice, around 1 August (Lughnasadh). It appears that the calendar was designed to align the lunations with the agricultural cycle of vegetation, and that the exact astronomical position of the Sun at that time was considered less important.
In medieval Ireland, Samhain remained the principal festival, celebrated with a great assembly at the royal court in Tara, lasting for three days, consistent with the Gaulish testimony.