You should aerate a lawn when soils are moist. Spring is a great time to aerate a lawn made of warm season grass. This is when the grass is actively growing and it will recover quickly from the process. The cooler season lawn is better aerated in the Autumn.
If you are not sure if you need to aerate, simply dig a section of turf out that is at least 1-inch square. If the brownish layer under the green, growing grass is an inch or more, then it is time to aerate. You can also just stab a screwdriver into the lawn (it won’t scream). If it is difficult to bury the tool to the hilt, it is a good time to aerate.