Busy org. on April 15th

IRS

["Think of the federal agency that collects taxes and might send you a refund—or an audit notice—around the spring deadline."]

This clue plays on the IRS being notoriously busy around April 15th, which is Tax Day in the U.S. when most federal income tax returns are due. "Org." is a common crossword abbreviation for "organization," and the date hints at the agency responsible for processing taxes. The answer is straightforward once you connect the deadline to the government group everyone associates with tax season.