The festival is, without a doubt, associated primarily with #Brigit / #Brighid, who among other things is the Irish Dawn Goddess. Dawn festival -> Dawn goddess. The "Saint" version was born at imbolc, at dawn - but the Christian hagiography makes a point of saying that she was born on the 8th of the Lunar Month. Perhaps, we could read this as a hint as to when her festival might have been calculated? The 8th of the next lunar month after... solstice? Equinox?

Imbolc was xtianised early, so IDK.