Lammas falls on August 1st. It is also called Lughnasagh after the Celtic God, Lugh. Another name for Lammas is August Eve, since the Celtic holiday started at sundown of July 31st, and lasted till sundown of August 1st.
This is the first harvesting Sabbat of the year. It concerns the harvest of grains, barley, wheat and oats, and corn in the United States. The traditional theme is the willing sacrifice of the God of the harvest, just as the wheat is cut down, so too is the God. Since Wicca is about the cycle of life and reincarnation, the God is reborn, even though He is sacrificed. In hollywood movies, like The Secret of Harvest Home, there is a man who takes on the role of the God and is, indeed, cut down. But that is Hollywood.
During a typical ritual, there might be a ritual drama of the death of the grain God, so that the people can eat and He can be reborn in the Spring. Then bread, the grain food, is passed out to be shared by the people, and thanks is given.