Downloading Not So Seecret Saturday

Multi-mc:

Download the NSSS multimc instance, a zip file which you can drag + drop
into multimc or a multimc-derived launcher of your choice.
If you run into a 'comparison method violates its general contract' crash,
try adding -Djava.util.Arrays.useLegacyMergeSort=true to your JVM args.
Also, please make sure you're running it on Java 5, 6, 7, or 8...

Launcher:

The official NSSS launcher is still getting some more polish before its released.
If you want to play NSSS on a platform that supports Java 5+ and LWJGL but doesn't have a native multimc port/fork, then the official NSSS launcher is the solution for you!

If you have a legit copy of Minecraft via your Microsoft account, the launcher will automatically update the game files with the latest version.
You will be able to play the game without an internet connection if you've run the launcher at least once while connected to the internet.

Not So Seecret Saturday server software:

The Not So Seecret Saturday server software is much-improved from Vanilla a1.1.2_01, however it's still not perfect! Hopefully it can provide a more-than-adequate experience.

If you want to run the server locally, or want to run it without the gui, here's how:
First, make sure you can use java from the command line. On linux and mac, this should automatically work, but on windows you might want to set up a PATH system variable.
Then download minecraft_server.jar to anywhere, then launch it as:
java -Xmx1024M -Xms1024M -jar a1.1.11_111-1_server.jar nogui
Alternatively, you can double-click the jar like an executable to run the server visually.