Most memorable Celtic games I attended in person:
1966-67 game vs Baltimore Bullets. Bill Russell's first year as player coach. I was seated about a dozen seats from Auerbach, who watched game from the stands. Bill Russell did not play that day, much to my disappointment (early load management, LOL). Bullets led by Gus Johnson and a rookie named Jack Marin, who went on to have a pretty good career. I don't remember who won!
1973 playoff games, 1 and 3, vs Knicks. We won game 1, and lost game 3, after Johnny Havlicek got nailed in the shoulder by a Dave Debusschere pick. Havlicek stayed in game and shot his free throws with his left arm, IIRC. We lost that game, and ended losing the next game on NY by some of the worst calls (thanks, Mindy Rudolph) in NBA history. Celtics were clearly the better team that year, but losing Hondo was too tough a hurdle to overcome.
1981 game 1 NBA finals vs Rockets, the game where Bird hit that incredible shot from behind the back board to win it. I was in the stands behind that basket, and could not appreciate the greatness of that shot live due to my angle. But was awesome being there. Frankly the most incredible moment of that game, for me, was right before the start of the game, where Bill Russell, Bob Cousy and, I think, Hondo, walked out onto the court to greet each other. The fans went absolutely crazy!!