I agree with Jess. Imbolc can be seen as the celebration of the first glimpses of Spring, Winter coming slowly to an end, the first flowers are coming out. IMHO not all Sabbats are necessarily linked to Gods or Wicca but to celebrate different times of the year like the first Harvest, the beginning of Winter, the glory of summer and sun.
For Witches, it symbolises the end of winter and the beginning of Spring, the start of new life, and new beginnings. You can dedicate it to Brigid, or to any of the other Spring Deities, or even just to Nature and the Earth if you so choose.