A proposition is an expression that have a definite truth value.
Thus "Napoleon is a penguin" is a _proposition_ , because it has a truth-value, and it is _false_.
Expression with indexicals are more complicated, because they need a "context" to be understood.
If I (mauro) am uttering it, because I (mauro) am not a penguin, then the expression "I'm not a penguin" is _true_.
The same (presumibely) if it is uttered by you (null).
But if the statement is uttered by Mumble (the protagonist of Happy Feet), in this case it is _true_.
* * *
Thus, following Demosthene's comment, the statement :
> “If Tuesday is a day of the week, then I am a penguin”
can be _true_ , if uttered by Mumble.