Complex sentence: Typically, this will refer to the album that was announced as number one on the Sunday before 25 December--when Christmas Day falls on a Sunday itself, the official number one is the one announced on that day's chart.
Split into simpler sentences: Typically, this will refer to the album that was announced as number one on the Sunday before 25 December. When Christmas Day falls on a Sunday itself, the official number one is the one announced on that day's chart.