Ubuntu itself continues to not participate in these development milestones in favor of early adopters, developers, and testers just using the latest daily ISOs. However, for others, their 16.04 Alpha 1 are out today. As of writing, Lubuntu, Ubuntu MATE, and Ubuntu Kylin are out with their 16.04 Alpha 1 releases while Xubuntu and Kubuntu are noticeably absent at the moment.
Ubuntu 16.04 "Xenial Xerus" will be officially released in April as the distribution's next Long-Term Support (LTS) release. More details via the mailing list announcement.