Complex sentence: The earliest known Christmas in July celebration was in July 1933 at Camp Keystone, a girl's summer camp in North Carolina, with a Christmas tree, gifts, and a visit by Santa Claus.
Split into simpler sentences: The earliest known Christmas in July celebration was in July 1933 at Camp Keystone (a girl's summer camp in North Carolina). This included a Christmas tree, gifts and a visit by Santa Claus.