Advice for directing further learning

I've stumbled across a problem that I can't get my head around and am not sure what to search for to learn how to resolve it. Can anyone help name a technique or method which I can read up on?

I'm trying to access a propety of an object which changes depending on the output of a function. This is a simplified example:

var days = {
  Monday: {hour: 6, minute: 30},
  Tuesday: {hour: 7, minute: 0},
};

var today = function() {
  return "Tuesday" // simplified for this example - not sure if return is best approach
};

console.log(days[today].hour);

Espruino says:

Uncaught Error: Cannot read property 'hour' of undefined
 at line 1 col 24
console.log(days[today].hour);
                       ^

Do I need to get today to return a string instead? Any help would be much appreciated.

This is a situation where I don't know enough to help myself :)