Hey Fabio,
This is a great question that I think a lot of businesses face this issue (maybe less drastically than you do though). From what you've described about your business it seems like your unique value proposition is that you sell an item associated with a holiday. This is also your issue but one that I think is solvable as well. The simple solution I see is that you should expand your product line to cover more holidays. Specifically I think you might be able to sell products for valentines day, independence day, thanksgiving, mother's day, father's day, hanukkah, new years, and potentially others. Your current business is a great opportunity to create fun clothing options for new holidays, take advantage of that.