From the Wikipedia:
Lammas Day (Anglo-Saxon hlaf-mas, 'loaf-mass') is a holiday celebrated in some English-speaking countries in the Northern Hemisphere, usually between 1 August and 1 September. It is a festival to mark the annual wheat harvest, and is the first harvest festival of the year. On this day it was customary to bring to church a loaf made from the new crop, which began to be harvested at Lammastide, which falls at the halfway point between the summer Solstice and Autumn September Equinox.
The loaf was blessed, and in Anglo-Saxon England it might be employed afterwards to work magic: a book of Anglo-Saxon charms directed that the lammas bread be broken into four bits, which were to be placed at the four corners of the barn, to protect the garnered grain.
The stamp was issued in 1981, and belongs too a 4-stamp series about folklore (see the stamps here).