The Celtic festival associated with St. Brigid and traditionally linked with fairy activity is Imbolc.
Imbolc is a Gaelic seasonal festival observed on 1 February, roughly halfway between the winter solstice and spring equinox. It marks the beginning of spring and was historically connected with lambing, milk, fertility and the return of agricultural life. In Christian Ireland, the date became the feast of St. Brigid.
The festival’s customs include weaving Brigid’s crosses, visiting holy wells and making a Brídeóg, a doll or figure representing Brigid. These practices blend Christian devotion with older seasonal traditions associated with the goddess Brigid. Imbolc is also known as Imbolg or St. Brigid’s Day.
Bealtaine and Samhain are the festivals most explicitly associated in many sources with heightened fairy activity, so they are common distractors. However, the reference to St. Brigid identifies Imbolc as the intended answer.