There are different ways to interpret this:

* "Conjunction": This means when two or more planets appear close together in the sky from our perspective on Earth. This happens relatively frequently, with the last notable conjunction being the Great Conjunction of Jupiter and Saturn in December 2020.

* "Syzygy": This refers to a precise alignment of three celestial bodies, such as the Sun, Earth, and another planet. This happens more rarely, with the last one involving the inner planets (Mercury, Venus, Mars, and Earth) occurring in October 2015.

* "Perfect alignment": This is a more theoretical idea, as planets in our solar system don't align perfectly in a straight line. There hasn't been a true "perfect alignment" in recorded history, and it's unlikely to happen for many thousands of years.

So, to give you a precise answer, I need you to clarify what kind of planetary alignment you're interested in!