Saint Barbara is patron Saint of Rethymno and in the old town, near the library is the Church named after her. On the 4th of December is the name day of St.Barbara and their are processions in the old town from an early hour.
Also bear in mind that Saint Barbara day is an official holiday for the municipality of Rethymnon. This means that public and private sector services will be closed for the day. As the 4th is on a Thursday it also means that there will be no market in the Marina.
I think that kafenions, bars etc are allowed to stay open. So I don't think it will be an issue, but I'm not 100% sure. Anyway, it's a nice walk along the seafront at this time of the year.