Spring aerating is usually discouraged because the aeration holes provide a perfect spot for weed seeds to germinate. Weeds seeds (especially crabgrass) are the first seeds to germinate in the spring, aerating the lawn just stirs them up and gives them an ideal home.
Late spring aerating may be beneficial if the thatch in the lawn is excessive (over 1"), the ground is extremely compacted, or there is excessively thin turf. Later in the spring is best, when weeds have established but before they flower and go to seed (around Memorial Day).
The best time to aerate is in the fall, when the temperatures have cooled off, the weed pressure is minimal, and grass is actively growing.
Aerating is primarily done to relieve compaction and reduce thatch. If spring overseeding is the goal, slice seeding can be done or seed can be sown and topdressed to provide adequate germination conditions.