Stranded aluminum on outdoor light switch
There are two separate questions here:
Timer Installation
This particular timer has connections for:

Line - This should be the hot wire coming into the existing switch.
Load - This is the currently switched hot wire going from the switch to the lights.
Neutral - This is NOT normally connected on a plain (not timer, not dimmer, not &quot;smart&quot;) switch. In newer installations it is required to be in the switch box. But it might not be in an older installation, in which case you have to find it, if it is available, which it might not be!
Traveler - This can be ignored, unless you have a 3-way switch (a second switch somewhere else controls the same lights as the switch you are replacing).

So the basic timer installation is easy, provided you have an accessible neutral.
Aluminum Wiring
Aluminum wiring is generally not used for low current (e.g., 15A and 20A circuits for residential lighting and receptacles). Depending on the type of wire, it may be &quot;OK, but need to make sure it is compatible with anything you connect it to&quot; or it may be &quot;really nasty stuff that should be ripped and replaced.&quot; I can't guess what you have. A few things that might help us figure this out are:

How old is your house? Where are you located (city/state/country)?
Is there any writing on the outside of the wires (i.e., codes that might tell us exactly what kind of wire you have)?
Are all the wires in your house aluminum, or just a few?

In addition, pictures of the current switch with its wires attached, as well as the model # of the new timer, may be helpful in figuring out what to do.