The last year before 2026 when a transit of Venus occurred was 2012.
The 2012 event took place on 5–6 June, depending on the time zone. Venus crossed the face of the Sun as a small dark disk, an alignment visible only when Venus passes between Earth and the Sun at the correct point in its inclined orbit.
Venus transits are exceptionally rare because Venus's orbital plane is tilted by about 3.4 degrees relative to Earth's. Most inferior conjunctions therefore pass above or below the Sun from our viewpoint. When the geometry does line up, transits usually arrive in pairs eight years apart: the previous pair occurred in 2004 and 2012.
The event also had major historical importance. Observations of earlier transits, especially those in the eighteenth and nineteenth centuries, helped astronomers estimate the astronomical unit—the Earth–Sun distance—through parallax. The 2012 transit was the last one of the twenty-first century, so the alternatives 2004, 1982, and 1999 do not fit the question: 2004 was the immediately preceding transit, while 2012 was the most recent year.