Lammas is the first Pagan harvest festival. It celebrates the first fruits (berries) and bread.
The date varies according to which tradition one follows. To me Lammas is always August 2nd.
Some people celebrate all the Quarter Sabbaths on 1st, and as the day begins by nightfall, the celebration is biggest on the Eve of the feast day - as we now have "Hallows E'en" or Halloween or New Year's Eve - not 1.1. but 31.12. In Finland we celebrate Christmas eve, not Christmas day, and we celebrate 1st of May on the Walburgis Eve - last of April. They will celebrate Lammas on August 1st and especially Lammas Eve, July 31st.
The astrological date for Lammas 2010 is August 8th, because the Sun will be in 15 degrees Leo.
Some celebrate the Quarter Sabbaths on the exact midpoint of Solar Sabbaths, and the midpoint of Summer Solstice (June 21) and Autumn Equinox (Sept 23) is Saturday, 7th of August.
Some celebrate Lammas according to the Old Ways, and that is at the first New Moon of August - that will be 10th of August.
Some celebrate it by the Full Moon closest to August 1st, and that will be 26th of July.