The significance might lie in the way the shirts are described in the story. If they are old, tattered shirts, they could symbolize the passage of time and the memories associated with past Christmases. If they are brand - new, shiny shirts, they could represent the new beginnings and hope that Christmas often brings. It all depends on how the story is crafted around these shirts.
Well, without more context, it's hard to say. But it could be that the shirts in the story are a symbol of something. Maybe they represent the togetherness of a family during Christmas, as they all wear matching shirts. Or perhaps they are a reminder of a past Christmas event related to the shirts.