Where am I? When am I? Why am I here?

You are exactly when and where you need to be.

This is still Yestertide.

On the day of the Great Storm.

In other words, the day the aliens came (no: will come) to Yestertide.

And you're here to stop them.

All you need to do is make sure the lighthouse light is off, so that there's nothing to attract the aliens to this landing site.

Then hopefully they will follow that new plan you uploaded and travel far, far away from here, never to return.

Good luck!