Complex sentence: Acting Governor Denver S. Dickerson and his family were the first residents when they occupied the mansion during July 1909 which was first opened to the public during an open house New Year's Day, 1910.
Split into simpler sentences: Acting Governor Denver S. Dickerson and his family were the first residents when they occupied the mansion in July 1909. The building was first opened to the public during an open house New Year's Day, 1910.