It was the afternoon of Jan. 13, 24 hours after center Dove Cowens announced that he was ending his 64-day, 31-game sabbatical and returning to the Boston Celtics, 18 hours after guard Charlie Scott celebrated the good news by breaking his forearm hideously in two places.

Celtic publicist Howie McHugh sat at grimy old Boston Garden on another gloomy day of the bone-chilling winter, philosophizing. "It's been like 'Mary Hartman, Mary Hartman' around here," he said, and that pretty well sums up the 1976-77 season of the defending National Basketball Association champions.

The Celtics play perhaps the pivotal game of their soap opera season tonight against the Philadelphia 76ers at the Spectrum in Philly. The winners take a 3-2 lead in the NBA's best-of-seven Eastern Coference semifinal playoff series. Game 6 is at Boston Friday night and Game 7, if necessary, at Philly on Sunday afternoon.

The Celts have sentiment and tradition on their side. They have played more playoff games (274) and won more titles (13) than any other team in NBA history.

But the oft-rhapsodized pride and mystique that always seem to elevate the ment in green at playoff time may not be enough to counteract Philadelphia's home court advantage and enormous one-on-one skills in this fascinating, emotional series, which many feel matches the best team against the best talent in basketball.

If the Celtsl do pull out two of the last three games - and don't bet all your leftover St. Patrick's Day cards or a high choice in the leprechaun draft against them - it will be a tribute to their resilience and the inspiration of the ghosts of playoffs past.

This has been a trying season for the Celts, who finished with a regular season record of 44-38, six games behind the 76ers in the Atlantic Division, but had Cowens and Scott in the lineup together for only 14 games.

When their highlights film is made the regular season can be capsulized by focusing on four days:

Nov. 10 - Having won their first four games and lost the next four, the Celtics got the unkindest cut of all from within as Cowens announced he was taking the NBA's first "leave of absence, without pay." Big Red, the 6-8, 230-pound tower of strength in the middle of the Big Green Machine, said he had "lost his enthusiasm," the most important element in his and the Celtics' style of play.

Jan. 12 - After meeting with another persuasive redhead, Arnold Auerbach, mastermind of the Boston dynasty as coach (1950-65), then general manager and president, Cowens announced he had regained his enthusiasm and would suit up against Portland two nights later.

That evening Scott, then the Celts' second leading scorer (19 points per game), collided with a basket support, fractured his arm, and was gone for the next 38 games.

Feb. 4 - Out of [WORD ILLEGIBLE] with the club slumping badly, coach Tommy Heinsonn switched Sidney Wicks, a major disappointment as a replacement for the departed Paul Silas (shipped to Denver after acrimonious contract negotiaions and a holdout), to sixth man. This is a position as vital in the traditional Celtic way as any of the five starting positions.

April 3 - Scott, his arm mended sooner than expected, returned to the lineup for the last eight days of the season. The Celts beat the 76ers, 96-90, on national TV, erasing a major psychological hangup.

The Sixers had won their previous theree regular-season meetings decisively, but never when Cowens and Scott were in harness together. A message emanated loudly from the Boston dressing room: "You haven't seen the Celtics unless you've seen us lately."

The return of Scott, a good shooter who goes to the hoop and plays much better defense than anyone thought he could before he came to Boston for Paul Westphal after the 1974-75 season, has helped. Cowens, nagged by injuries for a month after his unretirement, has regained that other-worldly look that signals he is determined to give the Celts the imtimidation they need from him, offensively and defensively, under the hoop.

John Havlicek, the 37-year-old captain whose 17 years as a Celtic have spanned the first dynasty (11 championships in 13 years, 1957-69), reconstruction, and the two titles the last three years, is still the playmaker who orchestrates the Celtics as surely as Arthur Fiedler directs the Boston Pops. He is, historically and artistically, the link to the glorious heritage, the complet player and consummate team man.

JoJo White, recently called by Heinsohn the most consistent offensive producer in Celtic history, has scored in double figures every game this year, making his contributions, in traditional style, with a minimum of fuss and fanfare.

But it was the much-maligned Wicks' performance as sixth man, allowing his ineffective-off-the-bench former UCLA teammate, Curtis Rowe, back into the starting lineup, that helped spark the revival even before Scott came back.

The sixth man has been paramount in the Celtics philosophy ever since Frank Ramsey, whose importance was recalled in halftime ceremonies last Sunday as the Celts commemorated the 20th anniversary of their first championship. Ramsey scored the winning basket ain the second overtime of the seventh game of the 1957 playoffs against the St. Louis Hawks.

Auerbach, in his recently published autobiography, explained his "sixth man" theory; "Psychologically, as soon as you pull one of your five starters out of the game, the other team is going to let down just a bit. That's when I wanted a guy like Ramsey or Havlicek to get out there and run them into the ground.

"All round the league, everybody starts his best. Suppose I don't start my best. Suppose I start 80 per cent of my best. Now, after five, six seven minutes go by, it's time to substitute. Their 100 per cent is getting tired, and so is my 80 per cent. In goes their sub and in goes my sixth man. What happens. They've decreased their proficiency while I've increased mine.

"It doesn't always work, of course. It depends on your personnel. But it's a good concept."

Wicks, a 6-9, 225-pounder who got bunches of "cosmetic" points and rebounds in four seasons at Portland - impressive on the stat sheets, but not critical in winning games - has had much more impact off the bench, beating up fatigued starters, than he did in battling fresh troops. Meanwhile Rowe, whose limitations on both offense and defense have been the other big disappointment, has been better as a starter.

Wicks has not exactly made people forget the offensive rebounding, muscle, team play and hustle of Silas and the retired Don Nelson, Satch Sanders, Jim Loscutoff and others before them.

But he has come a long way since Scott - an erstwhile one-man band who got religion when he put on the green uniform - reportedly pointed an accusing finger at Wicks during a disastrous December road trip and said, "The reason we aren't winning is because that guy ain't a Celtic."