Complex sentence: He had prepared for his entry into law by accumulating the names and addresses of everyone who could be a potential client and on October 11th, he mailed each of them a notice that he was starting the practice of law.
Split into simpler sentences: He had prepared for his entry into law by accumulating the names and addresses of everyone who could be a potential client. On October 11th, he mailed each of them a notice that he was starting the practice of law.