I wish I knew for certain the details of the first game I ever attended, but I don't. I believe it was May 23, 1967,. but I can't confirm. I remember attending a game where the Angels won 5-1 at some point early in my life (whether it was the first game or not, I don' t know) and I thought it was against either the Royals or the White Sox. And the only box score that matches that is May 23, 1967 when the Angels beat the Royals 5-1. It actually bugs me a little that I don't know. I know I saw Rich Reichard, Jay Johnston and Jim Fregosi play and all 3 were on that roster.
The earliest game I ever attended that I can confirm 100% is May 9, 1970. I can confirm that because it was "bat night" and I vividly remember Andy Messersmith beating the Yankees 11-3. I've been able to verify that one through news articles and the box score.
Two other games I remember specifically were Angels facing off against Vida Blue on July 4, 1971. Vida Blue pitched a gem.
And the Pittsburgh Pirates beating the Dodgers at Dodger Stadium on June 9, 1972. It was the first and only time I ever saw Roberto Clemente play in person.