The lustration was originally a sacrifice for expiation and purification offered by one of the censors in the name of the Roman people at the close of the taking of the census. The sacrifice was often in the form of an animal sacrifice, known as a suovetaurilia.
These censuses were taken at five-year intervals, thus a lustrum came to refer to the five-year intercensal period.
Livy reports that the first lustration occurred at the conclusion of the first census, conducted by king Servius Tullius in the 6th century BC by the lustration of the full Roman army.
This regular five-year interval between censuses was maintained in the early part of the Roman Republic, which lasted from ca. 509 BC–27 BC, but later became more irregular. The regular interval was restored by Octavianus Augustus in 28 BC after a "41 year gap", as attested by the Res Gestae Divi Augusti. At the time of the census, new senators were also appointed to the Roman Senate.
Source: Livy, Ab urbe condita, 1.44
"The work of the census was accelerated by an enactment in which Servius denounced imprisonment and even capital punishment against those who evaded assessment. On its completion he issued an order that all the citizens of Rome, knights and infantry alike, should appear in the Campus Martius, each in their centuries. After the whole army had been drawn up there, he purified it by the triple sacrifice of a swine, a sheep, and an ox. This was called “a closed lustrum,” because with it the census was completed. Eighty thousand citizens are said to have been included in that census. Fabius Pictor, the oldest of our historians, states that this was the number of those who could bear arms. To contain that population it was obvious that the City would have to be enlarged. He added to it the two hills—the Quirinal and the Viminal—and then made a further addition by including the Esquiline, and to give it more importance he lived there himself. He surrounded the City with a mound and moats and wall; in this way he extended the “pomoerium.” Looking only to the etymology of the word, they explain “pomoerium” as “postmoerium”; but it is rather a “circamoerium.” For the space which the Etruscans of old, when founding their cities, consecrated in accordance with auguries and marked off by boundary stones at intervals on each side, as the part where the wall was to be carried, was to be kept vacant so that no buildings might connect with the wall on the inside (whilst now they generally touch), and on the outside some ground might remain virgin soil untouched by cultivation. This space, which it was forbidden either to build upon or to plough, and which could not be said to be behind the wall any more than the wall could be said to be behind it, the Romans called the “pomoerium.” As the City grew, these sacred boundary stones were always moved forward as far as the walls were advanced."
Source: Wikipedia, The Free Encyclopedia, Lustrum.