The holiday season is the gift that keeps giving for comic book artists. From the earliest days of the four-color medium there have been Christmas and Santa Claus covers that nudge sales and cast a spell over collectors. At Marvel Comics, the holiday cover is such a set tradition now that the challenge is to find a concept and composition that feels truly original and not...ummmmm...''re-gifted.'' That's not a problem during other parts of the calendar: Valentine's Day motifs don't get much love, for instance, and no one expects an Easter Bunny cover to deliver sales bounce. So, with the season upon us, here's a selected look back at some of Marvel's modern history of holiday heroics.