No later than 20 days after the end of the quarterly period beginning on the first day of January, April, July, and October of each year in which a registrant is registered under section 1603 of this title, or on the first business day after such 20th day if the 20th day is not a business day, each registrant shall file a report with the Secretary of the Senate and the Clerk of the House of Representatives on its lobbying activities during such quarterly period. A separate report shall be filed for each client of the registrant.