Everyone sends a Christmas/ Hannukah/ Kwanzikah/ Winter Commercial Splurge Celebration cards to their customers. You aren’t going to stand out or make any sort of impression by sending out cards in the winter.
Instead, why not send out a Valentines Day card with “We Love Our Customers” or “We’re grateful for such sweet customers” and include a bag of gummy hearts or something. It’s not expensive, but it will stand out.
Some companies have started sending out Thanksgiving cards but that’s not very wide-spread yet so I’d encourage you to consider this holiday as well. Bonus points if you write a handwritten specific reason you’re thankful for each customer on their card. Bonus as well if it includes a coupon, a small free gift or a holiday recipe/tip.
Why not consider some other unusual card seasons? The holiday greeting companies have made every day a special holiday. You can look them all up by visiting http://www.holidayinsights.com/moreholidays.
Also consider the season and your customers. If you have a lot of teachers for customers you might send some sort of encouragement or useful information at the end and beginning of the school year. Every customer would appreciate a ‘de-stress your day’ card around tax season. Maybe you could make a ‘voting line survival pack’ to send during voting season.
The point is to not use cards as some sort of bland obligatory gesture, but as a genuine consideration of your customers. The more real you dialogue with them and the less conventional, the better your result!