Add peas to a large bowl and cover with water and soak overnight.
The next day, drain the peas and add to a large soup pan and add 4 cups water, carrots, onion, celery and bay leaves.
Bring to a boil and simmer uncovered for 45 minutes, or until most of the liquid has evaporated and the peas mash easily. Turn off the heat and mash, leave a few peas for texture.
Stir in butter, salt and pepper, taste for seasoning and adjust to taste.