In Italy, red underwear is traditionally worn on New Year's Eve to symbolize good luck.
The custom is especially associated with welcoming the new year in red garments, with red underwear often described as bringing luck, love, or prosperity. Contemporary versions are usually playful, but the practice belongs to a wider set of European New Year customs that assign symbolic powers to colors and clothing.
In popular accounts, the garment may be expected to be new, and some versions add rules about receiving it as a gift or discarding it afterward. These details are not universal and can vary by region or family.
The custom should not be confused with Italy's New Year's food traditions, such as lentils, which are associated with money and prosperity. The question concerns the red clothing practice, not a particular dish.