This story is borrowed from The Guideposts Christmas Treasury, which is currently out of print.

On Christmas Eve, as usual, George Mason was the last to leave the office. He stood for a moment at the window, watching the furrying crowds below, the strings of colored Christmas lights, the fat Sant Clauses on the street corners. He was a slender man in his late thirties, this George Mason, not conspicuously successful or brilliant, but a good executive - he ran his office efficiently and well.
Abruptly he turned and walked over to a massive safe set into the far wall. He spun the dials, swung the heavy door open. A light went on, revealing a vault of polished steel as large as a small room. George Mason carefully propped a chair against the open door of the safe and stepped inside.
He took three steps forward, tilting his head so that he could see the square of white cardboard taped just above the topmost row of strongboxes. On the card a few words were written. George Mason stared at those words, remembering...
Exactly one year ago he had entered this selfsame vault. He had planned a rather expensive, if solitary, evening; had decided he might need a little additional cash. He had not bothered to prop the door; ordinarily friction held the balanced mass of metal in place. But only that morning the people who serviced the safe had cleaned and oiled it. And then, behind George Mason's back, slowly, noiselessly, the ponderous door swung shut. There was a click of springlocks. The automatic light went out, and he as trapped - entombed in the sudden and terrifying dark.
Instantly, panic seized him. He hurled himself at the unyielding door. He gave a hoarse cry; the sound was like an explosion in that confined place. In the silence that followed, he heard the frantic thudding of his heart. Through his mind flashed all the stories he had heard of men found suffocated in timevaults. No timeclock controlled this mechanism; the safe would remain locked until it was opened from the outside. Tomorrow morning.
Then the sickening realization struck him. No one would come tomorrow morning - tomorrow was Christmas Day.
Once more he flung himself at the door, shouting wildly, beating with his hands until he sand on his kneew exhausted. Silence again, highpitched, singing silence that seemed deafening.
George Mason was no smoker; he did not carry matches. Except for the tiny luminous dial of his watch, the darkness was absolute. The blackness almost had texture; it was tangible, stifling. The time now was 6:15. More than 36 hours would pass before anyone entered the office. Thirty-six hours in a steel box three feet wide, eight feet long, seven feet high. Would the oxygen last, or would...
Like a flash of lightening a memory came to him, dim with the passage of time. What had they told him when they installed the safe? Something about a safety measure for just such a crisis as this.
Breathing heavily, he felt his way around the floor. The palms of his hands were sweating. But in the far right, just above the floor, he found it: a small, circular opening some two inches in diameter. He thrust his finger into it and felt, faint but unmistakable, a cool current of air.
The tension release was so sudden that he burst into tears. But at last he sat up. Surely he would not have to stay trapped for the full 36 hours. Somebody would miss him, would make inquiries, whould come to release him....
But who? He was unmarried and lived alone. The maid who cleaned his apartment was just a servant; he had always trated her as such. He had been invited to spend Christmas Eve with his brother's family, but children got on his nerves, and expected presents.
A friend had asked him to go to a home for elderly people on Christmas Day and play the piano - George Mason was a good musician. But he had made some excuse or other; he had intended to sit at home with a good cigar, listening to some new recordings he was giving himself for Christimas.
George Mason dug his nails into the palms of his hands until the pain balance the misery in his mind. He had thrown away his chances. Nobody would come and let him out. Nobody, nobody...
Marked by the luminous hands of the watch, the leaden-footed seconds ticked away. He slept a little, but not much. He felt no hunger, but he was tormented by thirst. MIserably the whole of Christmas Day went by, and the succeeding night....
On the morning after Christmas the head clerk came into the office at the usual time. He opened the safe but did not bother to swing the heavy door wide. Then he went on into his private office.
No one saw George Mason stagger out into the corridor, run to the water cooler, and drink great gulps of water. No one paid any attention to him as he descended to the street and took a taxi home.
There he shaved, changed his wrinkled clothes, ate some breakfast and returned to his office, where his employees greated him pleasantly but casually.
On his way to lunch that day he met several acquaintances, but not a single one had noticed his Christmas absence. He even met his own brother, who was a member of the same luncheon club, but his brother failed to ask if he had enjoyed Christmas.
Grimly, inexorable, the truth closed in on George Mason. He had vanished from human society during the great festival of brotherhood and fellowship, and no one had missed him at all.
Reluctantly, almost with a sense of dread, George Mason began to think about the true meaning of Christmas. Was it possible that he had been blind all these years, blind with selfishness, with indifference, with pride? Wasn't Christmas the time when men went out of their way to share with one another the joy of Christ's birth? Wasn't giving, after all, the essence of Christmas because it marked the time God gave his own Son to the world?
All through the year that followed, with little hesitant deeds of kindness, with small, unnoticed acts of unselfishness, George Mason tried to prepare himself....
Now, once more, it was Christmas Eve.
Slowly he backed out of the safe, closed it. He touched its grim steel face lightly, almost affectionately, as if it were an old friend. He picked up his hat and coat, and certain bundles. Then he left the office.
There he goes now in his black overcoat and hat, the same George Mason as a year ago. Or is it? He walks a few blocks, then flags a taxi, anxious not to be late. His nephews are expecting him to help them trim the tree. Afterwards, he is taking his brother and sister-in-law to a Christmas play. Why is he so inexpressibly happy? Why does this jostling against others, laden as he is with bundles, exhilarate and delight him?
Prehaps the card has something to do with it, the card he taped inside his office safe last New Year's Day. On the card is written, in Goerge Mason's own hand: To love people, to be indispensable somewhere, that is the purpose of life. That is the secret of happiness.