I used to work at Wal-Mart, where everyday meant people felt they could treat you like garbage, but Christmas meant they figured you should be licking their boots clean for them.
When I go out at Christmas I have zero expectation that a worker will be helpful at all, because I assume that they've hit the end of their rope from having to deal with the rest of the idiot shoppers who've gone through that day. So I take a different tact, and I simply don't go out shopping in the Christmas season unless I'm in a good enough mood to try to improve the shopping experience for the service staff. Big smiles, questions about their day, etc. I get damn great customer service almost everywhere I go.