Complex sentence: CSV also runs an annual "Make a Difference Day" on October 27th, promoting voluntary and community days of action throughout the UK, which is the UK's biggest day of direct volunteering in the community.
Split into simpler sentences: CSV also runs an annual "Make a Difference Day" on October 27th, promoting voluntary and community days of action throughout the UK. CSV's campaign to promote volunteering, CSV Make a Difference Day, is the UK's biggest day of direct volunteering in the community.