MONDAY: Another year, another children's Christmas party. I arrive to find Year 1 still engrossed in Raymond Briggs's cartoon about me, Father Christmas.

Change in side room, window with full view to the street. Costume feels as if it has not been washed since I and other Santas wore it last year. It's a cold day but the nylon outfit and beard, with elastic which digs into my ears, start to make me sweat. Realise, too late, that when I grabbed the black boots out of the car I took one of my wife's wellies. Result: two left boots. Hope nobody notices. Wait in corridor for my cue - jingling bells. Time for "Ho, ho, ho! Hello, everyone, I've just come from Lapland, my reindeer are outside. Did you hear them?"

The teachers call out their names and up they come: 70 children in 10 minutes. Hand them each a gift from a sack. No time for a chat. Probably a good thing since, when one boy tells me he wants a Playmobil pirate ship, I have a strong sense from his teacher that he will not be getting one - folks on the dole maybe.

Pick up the Raymond Briggstheme and wish them all a "blooming merry Christmas" - which they love, a sign, surely, that I am the genuine article - and I am off. Back into the side room, just in time to change and head off to pick up own children. Rush out, then stop: I left the outfit hanging on the back of the door facing the road. All the children will see it when they stream out. Race back, find a black bin-liner and cover costume. Phew. Secret safe for a while longer.

TUESDAY: Another school. My wife's class. Walk into the school office and introduce myself: "Hello, I'm Father Christmas." They don't bat an eyelid - the rest of the year if a stranger walked in claiming to be Santa they would call the police. Awestruck by the way my wife and a colleague are controlling and entertaining 70 or so five- and six-year-olds. Goes well. Every one of them comes and puts their arms round me. Feel oddly emotional. Whatever we think - and after 35 embraces my beard has slipped at an angle across my chin - for them I'm a wondrous figure.

Have the thought that being FC is the opposite of being a religious leader. They believe in me; I am the one who does not believe. On the way back to my sleigh I hug Mrs Blackhurst. One of the children mentions to the head how strange it was that Mrs Blackhurst seemed to know Father Christmas.

WEDNESDAY: Music in the road just as the children are going to bed. The Round Table is doing its pre-Christmas fund-raiser. Before we can stop them they charge out in their nightclothes. They stand, transfixed, as Santa waves from a truck. Our four-year-old looks pensive, then pronounces: "It's not the real one, he's got black hair." It's true, Santa has jet- black hair. On such incidentals do careers stand or fall.

THURSDAY: My wife tells me our nine-year-old is still a believer. We are going to my parents' for Christmas and he wants to know how Santa will know where he is.

I am shocked: in front of him we have been offhand about the great man. Makes you wonder what goes on in their heads: he must know that nobody can visit so many homes in such a short time. Some bright spark on the Internet has calculated there are 2 billion children in the world but since Santa tends not to go to Muslim, Hindu, Jewish and Buddhist households that reduces his workload to 15 per cent of the total, or 378 million. Reckon on a world average of 3.5 children per household and that is 91.8 million homes. To cover all those in 31 hours - thanks to different time zones, that is how long he has got - FC's sleigh must move at 650 miles per second and be capable of carrying 321,300 tonnes of toys, assuming each child receives nothing heavier than a medium-sized Lego set. On land, reindeer can pull no more than 300lb, so Santa needs 214,200 of the beasts.

FRIDAY: Still grappling with the magnitude of my persona, I notice that a Sheffield University professor has come up with the notion that Santa Claus was a junkie, or at least a figment of the crazed imaginations of people eatingmagic mushrooms, which made them believe humans could fly. A colleague brings me down to earth. His son is a firm disciple but is puzzled as to how Santa gets back up the chimney. That's more like it. Three more days to go, then the real work begins ...

Chris Blackhurst is one of Santa's thousands of helpers.