Oh no, I totally spaced on the i flag in my RegExp and needlessly duplicated effort. Good job.

This should have failed the validator because of array.indexOf(), shouldn't it?

Very basic kata, comments should be part of the description.

The default helper test in the CoffeeScript version has a syntax problem:

Test.assertSimilar(findOutChristmasWeekday '2013 12 25', 'Wednesday')
// turns into
Test.assertSimilar(findOutChristmasWeekday('2013 12 25', 'Wednesday'));

Valid options:

Test.assertSimilar((findOutChristmasWeekday '2013 12 25'), 'Wednesday')
Test.assertSimilar(findOutChristmasWeekday('2013 12 25'), 'Wednesday')

After all that work, way to drop my jaw.