Imbolic/Candlemas is the first of the 3 spring celebrations.
It is celebrated by many different paths. This is the day that Jesus was supposedly precented at temple. This was/is traditionally done 40 days after birth of a male child for a purification ceremony for the mother and a naming ceremony.
I personally don't know much about St. Brigid.
To me this is the first day of the "awakening" of the Earth Mother. Days are getting longer. Look around at the animals. Traditionally this is the time that many animals are reaching the end of their gestational period. Bear, and wolf cubs are being born. But they are still kept deep in the "womb" of the Earth Mother (in dens) until they are strong enough to leave.
The 2nd spring celebration is Ostara. This occurs on the Spring Equinox. (tends to happen around Easter and Passover) The Earth Mother is becoming more fertile soon new life will be springing forth. Flowers are blooming, trees are leafing out, more baby animals are seen. Rabbits have been a common theme because of fertility.
The 3rd spring celebration is Bealtaine-aka Mayday. The Earth Mother has reached full fertility. Now is the time of planting. Time to move the herds from winter pasture to summer pasture. The grasses have turned green and new life is begining to grow within the Earth Mother.
Now you have to remember, this is my take on the celebrations. I am an ecletic pagan who leans more towards Native American and Shamantic beliefs. I'm sure others here can give you more information on the Celtic and other paths of paganism.