Complex sentence: In English-speaking countries August 1 is Lammas Day, or loaf-mass day, the festival of the first wheat harvest of the year, on which day it was customary to bring to church a loaf made from the new crop.
Split into simpler sentences: In English-speaking countries, August 1 is Lammas Day (loaf-mass day), the festival of the first wheat harvest of the year. On this day it was customary to bring to church a loaf made from the new crop.