Aeration is the process of spiking your lawn to let air into the soil and this also assists moisture reaching the roots. Aeration is carried out generally to overcome problems of compaction that occur due to garden furniture, playing games on the lawn etc.
Blackfords recommend that aeration be carried out in Spring and Summer at approximately 6 week intervals when the soil is moist and preferably in cool showery weather on the areas where compaction has taken place. It may also be beneficial to aerate those areas where grass that appears to be lacking in vigour, which have moss in abundance and which turn brown in dry weather. It is best to use spiking implements that give a penetration of 100mm to 150mm. For small areas a garden fork can be used and a gentle rotation of the fork in the ground will enlarge the hole. However, for a large area unless your have lots of energy, stamina and time it may be wise to hire a spiking machine
Blackfords recommend at the end of the season that a more general and aeration be carried out possibly including hollow tining for heavy or waterlogged lawns but only after the lawn has been scarified.
Hollow-tining will probably only need to be carried out every 5 years or so and this involves the removal of a core of soil thus relievings compaction and allowing both air and water to penetrate. All tines should be removed from the lawn after this process and the holes should be filled with mixture of soil, peat, sand and well rooted organic matter.