The Writer’s Almanac provides this background on December 26:
Today is Boxing Day and St. Stephen’s Day in England, Canada, and several other countries. The origins of this national holiday are not certain, but it might come from an old custom of wealthy estate owners giving small gifts or money, wrapped in boxes, to their servants and those who worked for them. Servants were needed on Christmas Day to help with their masters’ holiday events, so they were often given a rest the next day. St. Stephen is honored today as the first Christian martyr, having been stoned to death for blasphemy.