Half Life Discord and Synergy Discord are running a Rebel Rush Event Today,

Take a look at your server list and filter by most players to easily find the "Rebel Rush" Labeled Servers

The Servers also do include the Synergy Rebel Rush Achievement.

For more information join the Synergy Discord

Important Preparation Steps for Rebel Rush Event
With the release of the 20th Anniversary game update.

Switch to the Legacy Build for Half-Life 2:

Right-click on Half-Life 2 in your Steam Library and select Properties.
Navigate to the Betas tab and select the steam legacy 'pre 20th anniversary' build.
This will update Half-Life 2, Episode One, Episode Two, and Lost Coast to the [steam_legacy] version.
Initialize the Game:

Launch Half-Life 2 and start a new game.
After the game begins, you can exit immediately (you do not need to watch the cinematic).
This step ensures compatibility with Synergy servers.
Update Synergy:

Make sure you are on the latest Development Branch of Synergy. Check for updates prior to the event.
Once these steps are completed, you’ll be ready to join the Rebel Rush event without issues. Let’s make this one to remember—see you there!