The planet with the most significant length of day compared to its year is Venus. A single day on Venus, which is the time it takes for the planet to complete one full rotation on its axis, lasts about 243 Earth days. In contrast, a year on Venus, or the time it takes to orbit the Sun, is approximately 225 Earth days. This means that a day on Venus is longer than its year, a unique characteristic among the planets in our solar system. Additionally, Venus rotates very slowly and in the opposite direction to most planets, resulting in unusual day-night cycles.