About 6,000 federal inmates whose long sentences were reduced last year will be released at the end of October, marking the start of the most substantial effort yet to reduce America’s gargantuan prison population.

The U.S. Sentencing Commission, an independent judicial agency that oversees federal sentencing guidelines, first voted in April 2014 to reduce federal drug guideline sentences in an effort to curb prison overcrowding and excessive punishments. But those amendments applied to about 70 percent of future drug sentences, and did not affect prisoners already serving federal prison sentences for drug convictions.

Amid public pressure from criminal-justice reformers, federal judges, and members of Congress, the commission subsequently voted that July 2014 to make the changes retroactive and allow current inmates to apply for re-sentencing. An estimated 40,000 and 45,000 federal inmates became eligible for lower sentences under the new guidelines. As of this August, 17,446 inmates had applied for the hearings, according to the Sentencing Commission. 13,187 of them, or about 75.6 percent, received sentence reductions under the new guidelines. The federal prison system currently houses over 205,000 inmates; over 1.2 million state prisoners will be unaffected by the change.

Technically, many of the soon-to-be-released inmates’ sentences are already complete. As part of its vote to make the guideline change retroactive, the commission delayed the initial release of prisoners for over a year until November 1, 2015 to give the courts and the Bureau of Prisons time to prepare for their reentry. Even if a re-sentenced inmate’s sentence ended before November, he or she would remain behind bars until that date. As a result, about 6,000 inmates whose sentences have already ended will be released on or around November 1, according to The Washington Post.

Although the number is evocative—the Post called it “the largest ever one-time release of federal prisoners”—the spike in releases simply clears out an accumulated backlog created by the one-year delay. Officials said the delay would allow for legal proceedings to assess the public-safety impact of each release. Each eligible inmate filed for re-sentencing separately, with the process negotiated between their legal counsel and the prosecutor, then approved by a federal judge.

The delay also allowed for prison officials to arrange for the inmates’ transition back into society. Many, if not all, of the inmates will transition to halfway houses or similar facilities upon their release from prison. “People will not just go to the street without any kind of preparation,” said Mary Price, the general counsel for Families Against Mandatory Minimums, a sentencing-reform advocacy organization. All federal drug prisoners are also required to serve a period of supervised release.

The retroactive sentencing reforms are one of the largest efforts yet to curtail mass incarceration in the United States. Other branches of government have also made tentative steps towards reform over the past year. Last week, a bipartisan group of senators released a long-awaited criminal-justice reform bill that aims to reduce mandatory minimums and expand prisoner re-entry programs. In July, President Obama commuted the sentences of 46 nonviolent drug offenders in federal prisons.

But the impact of mandatory-minimum sentences on the American prison system is immense and long-lasting. Although the first 6,000 inmates will be released over a three-day period because of the delay, the remainder of the re-sentenced inmates will slowly trickle out of prison as their sentences end throughout the next few decades.