In version 2015.3.930, it looks like the calendar.firstDay property is set to 1 (Monday) instead of 0 (Sunday) in the en-US culture definition. This is causing calendar and date picker widgets to start with Monday instead of Sunday. You can test this by running the following in your browser console:
// Returns 1 (Monday)
For now, the quick fix is just to change this in the kendo.culture.en-US.min.js file.