Receiving events from VS service in VSIX extension right from IDE startup
See the question and my original answer on StackOverflowA package can be set to autoload in a particular user interface context, for example, when a solution is open.
The ProvideAutoLoadAttribute attribute sets this context.
....
[ProvideAutoLoad(UIContextGuids80.SolutionExists)] // choose when you want it to be loaded
...
public class MyAutoloadedPackage : Package
{
...
}