So my problem was quite simple... syntax. Being very new to coffescript and spinejs I guess I was looking at too many things at once and missed the obvious.
The proper way to call this function from a coffeescript/eco template is:
<% day = cal.firstDay() %>
I was missing the parenthesis on the function call. This mistake should be easy to recognize if I run across the error again in the future. In fact it should have been obvious in the first place... again, too many new things.