The "DayPilotMonth is undefined" error can happen in two cases:
- The JavaScript code is not properly loaded (it's loaded using WebResources.axd reference).
- There is an syntax error inside the JavaScript code. That prevents the DayPilotMonth class from being defined.
I've tried the build 1389 and it seems that it's not caused by a syntax error (the demo works fine).
I would suggest that you try reloading the page using Ctrl+F5. If that doesn't help, try to look at the actual WebResource.axd content - Firefox + Firebug might help here. There will at least two WebResource.axd references: one defining DayPilotMonth (Month.js in the sources) and common definitions (Common.js in the sources).
It might render ASP.NET exception page instead of the JavaScript code in some cases. It can happen if you use URL redirection, authentication, etc.