Lammas,strictly speaking,was the "rest" period after the first harvest of the year;usually wheat. When the harvest was gathered in,it was often celebrated by the first bread of the new harvest. At the growth of Christianity it was usual to bake a special loaf for the church,and offer the bread up to God.
So,Lammas is celebrated with bread, wine or cider, and is a "rest" from work. Sometimes there was an offering of the new bread to "the Gods" for a good harvest the following year.