The pre-op test is run to ensure that your game runs smoothly and that all the required mods work for you. In addition to this, it also ensures you can check the gear for your assigned role and report back to management if it is incorrect.
How will I know when it is time to join?
You will be alerted to the start time by several different means. Via Discord, Teamspeak and the website.
In the Teamspeak channel the joining lobby will be given a "green light" to show the server is up and ready to join, here you will also find the map for the mission in the channel name.
You may be pinged on Discord to join, or a message put on so people can see.
The website shows dates/times for events if you are ever unsure.