LibreOffice uses a time-based release schedule, to produce the best quality free software. A time based release is one that does not wait for either features or bug fixes but is based (as much as possible) on time. This enforces discipline in introducing fixes, gives predictability, and allows more regular releasing. In addition, synchronizing a time-based release schedule with the wider free software ecosystem also offers huge advantages, by getting new features out to users as quickly as possible with a minimum of distribution cycle lag.
As a result, users get new major version every six months with a wide range of features, fixes, and enhancements. In addition, they get many pure bugfix micro releases. The first X.Y.0 release is intended for early adopters. More conservative users are advised to wait for a later X.Y.Z bugfix release.
TDF strongly suggests early adopters and tech savvy users to deploy the latest bleeding edge version (now LibreOffice 5.2.3, and soon LibreOffice 5.2.4), and more conservative individual users and enterprise users to deploy the longer tested previous version (LibreOffice 5.1.6, for enterprise users with the addition of professional support provided by the ecosystem). For testing and quality assurance purposes, though, the project maintains a repository of every older LibreOffice version: https://downloadarchive.documentfoundation.org/libreoffice/old/.