Forget Advent calendars, C-listers flicking on lights or teeth-numbingly saccharine TV ads. The official countdown to Christmas – festival of who can stick the most in their gob – is measured in eggnog lattes, mince-pie ice-creams and the ubiquitous Christmas sandwich. The appearance of the latter, the bright-red box in a sea of humdrum sarnies, can induce excitement in office workers akin to Santa himself sneaking into the chiller cabinet at Pret.

All the usual purveyors of lunch "al desko" now do their own take on the most holy of Yuletide traditions – the Leftover Sandwich. That dirty doorstop you somehow force down at 10pm on Christmas Day. It's not really a sandwich, more a bread-based receptacle for everything that survived lunch – turkey, stuffing, cranberry sauce, maybe even a rogue, bisected sprout. Everyone has their own preferences for perfection.

Even if it does contain cardboard turkey, questionable bacon and more cranberries than a cystitis sufferer's fridge, from now until 25 December, your old egg-and-cress faithful won't get a look in. Think of it as a warm-up to the eating marathon to come. Yes, the mayo drips down your chin, the cranberry stains your shirt and you'll feel nauseous after three bites, but just focus on the birth of Jesus happening in your mouth.

This year Marks & Spencer, not content with its "3 Course Christmas Feast" sandwich, is really celebrating with Festive Sushi, which is in no way just bog-standard sushi arranged in a Christmas tree-shaped box. And if you're not sure why you're spending £4 on your meat-and-two-bread, do remember that these purveyors are donating at least 3p per sarnie to charity. Oh, and try to steer clear of the on-pack calorie count. Why do you think Santa's so fat?

Who knows what other joys of Christmas will be marketed as limited-edition must-haves in years to come: Christmas-feast smoothies? Wine labelled Chateau de Snog-someone-inappropriate-at-your-office-party? Dad's farts in a can? You can't put a price on tradition.