The planet known for having the longest day is Venus. A day on Venus, defined as the time it takes to complete one full rotation on its axis, lasts approximately 243 Earth days. Interestingly, this rotation period is longer than a Venusian year, as the planet takes about 225 Earth days to complete one orbit around the Sun. Additionally, Venus has a very slow and retrograde rotation, meaning it rotates in the opposite direction to most planets in the solar system. This unique combination of long rotation and orbital dynamics contributes to Venus's distinctive day length and atmospheric conditions.