Question: During what season did an American professional basketball team based in Washington, D.C acquired Elvin Hayes from the Houston Rockets?

Context paragraphs:
Jack Marin: John Warren Marin (born October 12, 1944) is a retired American professional basketball player.  A 201 cm (6-foot, 7-inch) guard/forward from Duke University, Marin was named to the 1967 NBA All-Rookie Team and spent 11 seasons in the NBA (1966–1977), playing for the Baltimore Bullets, Houston Rockets, Buffalo Braves and Chicago Bulls.  The left-handed Marin was a two-time All-Star and scored 12,541 points in his career.  He is perhaps most remembered for being traded to the Rockets (along with future considerations) for Elvin Hayes on June 23, 1972.  He led the NBA in free throw percentage during the 1971–72 NBA season.

1972–73 Baltimore Bullets season: During the 1972–73 NBA season, the Bullets acquired Elvin Hayes from the Houston Rockets.  In the draft, the Bullets acquired Kevin Porter.  After a slow start, the Bullets had a strong December by posting a 10–4 record.

Washington Wizards: The Washington Wizards are an American professional basketball team based in Washington, D.C. The Wizards compete in the National Basketball Association (NBA), as a member of the league's Eastern Conference Southeast Division.  The team plays its home games at the Capital One Arena, in the Chinatown neighborhood of Washington, D.C.

Houston Rockets: The Houston Rockets are an American professional basketball team based in Houston, Texas.  The Rockets compete in the National Basketball Association (NBA), as a member of the league's Western Conference Southwest Division.  The team plays its home games at the Toyota Center, located in downtown Houston.  The Rockets have won two NBA championships and four Western Conference titles.  The team was established as the San Diego Rockets, an expansion team originally based in San Diego, in 1967.  In 1971, the Rockets moved to Houston.

List of Houston Rockets seasons: The Houston Rockets are an American professional basketball team based in Houston, Texas.  The Rockets play in the Southwest Division of the Western Conference in the National Basketball Association (NBA).  The team was established in 1967, and played in San Diego, California for four years, before moving to Houston, Texas.

List of Denver Nuggets head coaches: The Denver Nuggets are an American professional basketball team based in Denver, Colorado.  They play in the Northwest Division of the Western Conference in the National Basketball Association (NBA).  The Nuggets were founded as the Denver Rockets of the American Basketball Association (ABA) in 1967.  In 1974, in anticipation of moving to the NBA, the franchise held a contest to choose a new trademarked name for the team, as Rockets was already in use by the Houston Rockets and the name Nuggets won.  In 1976, the ABA folded, and the NBA decided to admit four ABA teams into the league, including the Nuggets, the San Antonio Spurs, the Indiana Pacers and the New York Nets.

Hofheinz Pavilion: Hofheinz Pavilion is a 8,479-seat multi-purpose arena on the University of Houston campus in Houston, Texas, currently undergoing renovation.  Located at 3875 Holman Street, it is home to the Houston Cougars men's and women's basketball teams as well as the women's volleyball team.  The building is named after Roy Hofheinz his late wife, Irene Cafcalas "Dene" Hofheinz after they donated $1.5 million to help fund construction.  Roy Hofheinz, known as Judge Hofheinz, was a UH alumnus and a Houston politician, businessman, and philanthropist.  The court is named for hall of fame and former Cougars coach Guy V. Lewis.  The arena also contains an alcove dedicated to Basketball Hall of Famer Elvin Hayes, a Cougar player in the 1960s and NBA star in the 1970s.  The arena opened in 1969.  Like many arenas of its kind, the seating bowl of Hofheinz Pavilion is dug into the ground so that one enters the building at the top of the bowl.  The arena has been renovated several times and is currently undergoing a major renovation to become the Fertitta Center, scheduled to reopen for the 2018–19 men's and women's basketball seasons.

Houston Rockets all-time roster: The Houston Rockets are an American professional basketball team based in Houston, Texas.  The team plays in the Southwest Division of the Western Conference in National Basketball Association (NBA).  The team was established in 1967, and played in San Diego, California for four years before being moved to Houston.  In the Rockets debut season, they won only 15 games.  However, after drafting Elvin Hayes first overall in the 1969 NBA Draft, they made their first appearance in the playoffs in 1969.  After Hayes was traded, Moses Malone was acquired to replace him.  Malone won two MVPs during his time in Houston, and he led the Rockets to the conference finals in his first year with the team.  He also took the Rockets to the NBA Finals in 1981, but they were defeated in six games by the Boston Celtics.  In 1984, the Rockets drafted Hakeem Olajuwon, who led them to the 1986 Finals in his second year, where they lost again to Boston.  In the next seven seasons, they lost in the first round of the playoffs five times.  They won their first NBA championship in 1994, led by Olajuwon, who won Finals MVP.  They repeated as champions the next year, and Olajuwon won Finals MVP once again.  To date, the Rockets have not advanced to the finals again.  The Rockets missed the playoffs from 1999–2003, and did not make the playoffs again until after they drafted Yao Ming in 2002.  Since then, the Rockets have had a winning season in all but two of the next 14 seasons and, led by James Harden, advanced to the conference finals in 2015.

List of Houston Rockets head coaches: The Houston Rockets are an American professional basketball team based in Houston, Texas.  They are a member of the Southwest Division of the Western Conference in the National Basketball Association (NBA).  They have played their home games at Toyota Center since 2003.  Founded by Robert Breitbard in San Diego, the team first joined the NBA in 1967 as an expansion team.  After four losing seasons, however, the team was sold to a group of investors based in Houston.  The group subsequently relocated the team to Houston, where the Rockets have remained for 37 seasons.  The franchise won its only two NBA titles in 1994 and 1995.

History of the Houston Rockets: The Houston Rockets are an American professional basketball team based in Houston, Texas.  The team plays in the Southwest Division of the Western Conference in the National Basketball Association (NBA).  The team was established in 1967, and played in San Diego, California for four years, before moving to Houston, Texas.

Supporting paragraphs: 1972–73 Baltimore Bullets season Washington Wizards
Answer: 1972–73 NBA season