The correct syntax for this is...
Session::set('variableName', $value);
For Laravel 5.4 and later, the correct method to use is put.
Session::put('variableName', $value);
To get the variable, you'd use...
Session::get('variableName');
If you need to set it once, I'd figure out when exactly you want it set and use Events to do it. For example, if you want to set it when someone logs in, you'd use...
Event::listen('auth.login', function()
{
Session::set('variableName', $value);
});