Beginning and end dates of each session of the U.S. Congress dating back to its conception are available at Senate.gov. For each numbered Congress, there are two sessions spanning the time between elections for U.S. representatives, usually starting in January of each year.
Congressional session dates describe the start and end dates used by Congress roughly on each end of a calendar year. A calendar outlining specific days during the session that the Senate and House of Representatives take their respective recesses is available at Congress.gov. In several years, the House of Representatives and the Senate ended their yearly sessions on different dates or at different times on the same date.