Answering this question requires an understanding of the differences between thatching and aerating.
Some thatch can be beneficial.
Also, an excessive thatch layer can become a breeding ground for insects and grass diseases that are detrimental to the lawn. Removing the excessive thatch layer is best done during the vigorous growing seasons of spring and early fall, and should be followed by a root stimulating fertilizer and watering. For the turf blends in our climate, the best time to aerate is also during the spring and early fall, again followed by an application of a root stimulating fertilizer and watering. As to which treatment should be done when, Brennan recommends aeration as part of regular lawn maintenance. I suggest thatching only when the thatch layer becomes excessive and significantly blocks nutrition absorption.