3 of the 4 major US sports use the 7 game format for their playoffs. While game 7s aren't hugely uncommon, it certainly is rare to see the same two teams meet in a decisive final game so many times. This will be the 6th game 7 contested between these two historical rivals.
Here's the history.
1965 Eastern Division finals: Celtics 110, 76ers 109
1968 Eastern Division finals: Celtics 100, 76ers 96
1977 Eastern Conference semifinals: 76ers 83, Celtics 77
1981 Eastern Conference finals: Celtics 91, 76ers 90
1982 Eastern Conference finals: 76ers 120, Celtics 106
So the Sixers are 3-2 all time in game 7s vs Boston. We'll be adding one more result to this list in just a few hours.