Another setting background by fixed calendar date - MMRIZE/MMM-CalendarExt3 GitHub Wiki

/* css/custom.css */

.CX3 .cell.month_12.date_25 {
  background-image: url("/modules/MMM-CalendarExt3/christmas.png");
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}