My immediate association with this word is Terry Pratchett's Discworld novels, where widdershins is one of the four directions (along with turnwise, hubwards and rimwards).
In this world it generally means anti-clockwise, the opposite way to the sun, or just the other way to the usual one. I also found a few interesting warnings about what can happen if you do things widdershins... walking widdershins round a church is very unlucky and might whizz you off to goodness-knows-where, and dancing that way round a ring of toadstools could result in falling under the power of fairy people.
Consider yourselves warned! And visit Carmen (although possibly not till Sunday) to see who else is playing this week.