Nova Launcher Betas include new features and fixes before they are released to all users. However they also are not as well tested and may not be as reliable. Betas are best for people who don't mind when things temporarily break a little bit.
Betas are distributed either via Google Play by opting in as a tester, or directly from this website.
To opt-in for Nova Launcher Betas from Google Play:
Installing betas does not require uninstalling your current version of Nova Launcher. Betas are compatible with your existing Nova Launcher Prime license and you will not lose Prime features by installing a beta.
For betas outside of the Play Store, install the latest Nova Launcher APK and enable automatic update checks and update from within the app via Settings > Check for updates > Menu > Beta
Nova Launcher Beta G+ Community
5.3-beta3 Jun 21, 2017
5.3-beta2 Jun 16, 2017
5.3-beta1 Jun 14, 2017
5.2 Jun 6, 2017
5.2-beta3 Jun 2, 2017
5.2-beta2 May 26, 2017
5.2-beta1 May 25, 2017
5.1.1 May 9, 2017
5.1 May 4, 2017
5.1-beta6 Apr 21, 2017